Deprecate support for Python 3.8

[mjcaley]

• Add Reno template
• Deprecate Python 3.8 #462

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (7afcdce) to head (97420c8). Report is 67 commits behind head on development.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #470 +/- ## ============================================= Coverage 100.00% 100.00% ============================================= Files 11 11 Lines 1194 1194 Branches 147 147 ============================================= Hits 1194 1194 ``` | [Flag](https://app.codecov.io/gh/mjcaley/aiospamc/pull/470/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Caley) | Coverage Δ | | |---|---|---| | [integrationtests](https://app.codecov.io/gh/mjcaley/aiospamc/pull/470/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Caley) | `63.14% <ø> (ø)` | | | [unittests](https://app.codecov.io/gh/mjcaley/aiospamc/pull/470/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Michael+Caley) | `100.00% <ø> (ø)` | | 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=Michael+Caley#carryforward-flags-in-the-pull-request-comment) to find out more.

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