Support `X | Y` and `list[T]` for older versions of Python

[brentyi]

Previously, X | Y was only available in Python >=3.10 and list[T] was only available in Python >=3.9. We added support for them in Python 3.7 & 3.8.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.89%. Comparing base (28082f8) to head (dac7b0a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #134 +/- ## ========================================== + Coverage 99.58% 99.89% +0.30% ========================================== Files 24 24 Lines 1951 1934 -17 ========================================== - Hits 1943 1932 -11 + Misses 8 2 -6 ``` | [Flag](https://app.codecov.io/gh/brentyi/tyro/pull/134/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/134/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%> (+0.30%)` | :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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