chore: add Python 3.13 build

[gruebel]

This PR

• a bit early, but next week the first beta release will come out and that marks no further feature development
• also increased the default version to Python 3.12 to align it with the version used during the release

[codecov[bot]]

Codecov Report

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

Project coverage is 97.32%. Comparing base (3b967a9) to head (08d6adb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #325 +/- ## ======================================= Coverage 97.32% 97.32% ======================================= Files 26 26 Lines 1198 1198 ======================================= Hits 1166 1166 Misses 32 32 ``` | [Flag](https://app.codecov.io/gh/open-feature/python-sdk/pull/325/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/open-feature/python-sdk/pull/325/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature) | `97.32% <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=open-feature#carryforward-flags-in-the-pull-request-comment) to find out more.

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[gruebel]

strange error, seems to be htach related, because I was able to create a 3.13 venv locally and run our tests there 🤔 will revisit it again with the first RC version in July.