Support Python 3.12 + PEP 695 (`type` statement, new type parameter syntax)

[brentyi]

• [x] Add Python 3.12 to CI + pyproject.toml
• [x] Add example for Python 3.12-style generics
• [x] Add support for type statement

[codecov[bot]]

Codecov Report

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

Project coverage is 99.89%. Comparing base (b43c668) to head (43ac008).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #135 +/- ## ======================================= Coverage 99.89% 99.89% ======================================= Files 24 24 Lines 1943 1945 +2 ======================================= + Hits 1941 1943 +2 Misses 2 2 ``` | [Flag](https://app.codecov.io/gh/brentyi/tyro/pull/135/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/brentyi/tyro/pull/135/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | `99.89% <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=Brent+Yi#carryforward-flags-in-the-pull-request-comment) to find out more.

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