Add workaround for Python<=3.10 `get_type_hints()` bug

[codecov[bot]]

Codecov Report

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

Project coverage is 99.70%. Comparing base (42b9454) to head (e336e16). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #157 +/- ## ======================================= Coverage 99.70% 99.70% ======================================= Files 24 24 Lines 2018 2019 +1 ======================================= + Hits 2012 2013 +1 Misses 6 6 ``` | [Flag](https://app.codecov.io/gh/brentyi/tyro/pull/157/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/157/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | `99.70% <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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