Fix `init=False` and forward references for attrs

[codecov[bot]]

Codecov Report

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

Comparison is base (f674d1d) 99.58% compared to head (ea8f330) 99.59%.

:exclamation: Current head ea8f330 differs from pull request most recent head 7611e1d. Consider uploading reports for the commit 7611e1d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #126 +/- ## ======================================= Coverage 99.58% 99.59% ======================================= Files 24 24 Lines 1948 1952 +4 ======================================= + Hits 1940 1944 +4 Misses 8 8 ``` | [Flag](https://app.codecov.io/gh/brentyi/tyro/pull/126/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/126/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | `99.59% <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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