Fix the way obsolete messages are stored

[tomasr8]

Closes #1124

Previously, the Catalog.obsolete dictionary would use the msgid as a key to store messages. When there are two messages with the same id but a different context, the latter one overwrites the former.

The fix is to use Catalog._get_key as the key, same as for _messages.

This might be considered a breaking change, however the only documentation for Catalog.obsolete is this:

The docs never say anything about how the keys are computed, so I think it should be safe to make this change (and perhaps document it too?)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.31%. Comparing base (f91754b) to head (efea1fe). Report is 8 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1132 +/- ## ========================================== + Coverage 91.26% 91.31% +0.05% ========================================== Files 27 27 Lines 4602 4618 +16 ========================================== + Hits 4200 4217 +17 + Misses 402 401 -1 ``` | [Flag](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | Coverage Δ | | |---|---|---| | [macos-12-3.10](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.08% <100.00%> (+0.01%)` | :arrow_up: | | [macos-12-3.11](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.01% <100.00%> (+0.01%)` | :arrow_up: | | [macos-12-3.12](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.23% <100.00%> (+0.05%)` | :arrow_up: | | [macos-12-3.13-dev](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `89.75% <100.00%> (+0.05%)` | :arrow_up: | | [macos-12-3.8](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.01% <100.00%> (+0.01%)` | :arrow_up: | | [macos-12-3.9](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.01% <100.00%> (+0.01%)` | :arrow_up: | | [macos-12-pypy3.10](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.08% <100.00%> (+0.01%)` | :arrow_up: | | [ubuntu-22.04-3.10](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.10% <100.00%> (+0.01%)` | :arrow_up: | | [ubuntu-22.04-3.11](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.03% <100.00%> (+0.01%)` | :arrow_up: | | [ubuntu-22.04-3.12](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.25% <100.00%> (+0.05%)` | :arrow_up: | | [ubuntu-22.04-3.13-dev](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `89.77% <100.00%> (+0.05%)` | :arrow_up: | | [ubuntu-22.04-3.8](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.03% <100.00%> (+0.01%)` | :arrow_up: | | [ubuntu-22.04-3.9](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.03% <100.00%> (+0.01%)` | :arrow_up: | | [ubuntu-22.04-pypy3.10](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.10% <100.00%> (+0.01%)` | :arrow_up: | | [windows-2022-3.10](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.22% <100.00%> (+0.01%)` | :arrow_up: | | [windows-2022-3.11](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.15% <100.00%> (+0.01%)` | :arrow_up: | | [windows-2022-3.12](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.37% <100.00%> (+0.05%)` | :arrow_up: | | [windows-2022-3.13-dev](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `89.89% <100.00%> (+0.05%)` | :arrow_up: | | [windows-2022-3.8](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.15% <100.00%> (+0.01%)` | :arrow_up: | | [windows-2022-3.9](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.15% <100.00%> (+0.01%)` | :arrow_up: | | [windows-2022-pypy3.10](https://app.codecov.io/gh/python-babel/babel/pull/1132/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel) | `90.22% <100.00%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-babel#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.