Add support for Python 3.13

[hugovk]

Still in alpha, but let's try supporting it already, we might find some bugs to report upstream before the big release.

• https://devguide.python.org/versions/
• https://peps.python.org/pep-0719/

[codecov[bot]]

Codecov Report

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

Comparison is base (ec9a971) 94.38% compared to head (e4fa7dd) 94.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #281 +/- ## ======================================= Coverage 94.38% 94.38% ======================================= Files 5 5 Lines 2404 2404 ======================================= Hits 2269 2269 Misses 135 135 ``` | [Flag](https://app.codecov.io/gh/jazzband/prettytable/pull/281/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband) | Coverage Δ | | |---|---|---| | [macos-latest](https://app.codecov.io/gh/jazzband/prettytable/pull/281/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband) | `94.34% <ø> (ø)` | | | [ubuntu-latest](https://app.codecov.io/gh/jazzband/prettytable/pull/281/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband) | `94.34% <ø> (ø)` | | | [windows-latest](https://app.codecov.io/gh/jazzband/prettytable/pull/281/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jazzband) | `94.30% <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=jazzband#carryforward-flags-in-the-pull-request-comment) to find out more.

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