Fix type narrowing edge case, bump to 0.7.1

[brentyi]

Fixes:

• https://github.com/nerfstudio-project/nerfstudio/issues/2862
• https://github.com/brentyi/tyro/issues/121

Needs tests!

[brentyi]

Well this is not right, but the general idea is there...

[codecov[bot]]

Codecov Report

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

Comparison is base (d660da2) 99.58% compared to head (13230d1) 99.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #122 +/- ## ======================================= Coverage 99.58% 99.58% ======================================= Files 24 24 Lines 1948 1949 +1 ======================================= + Hits 1940 1941 +1 Misses 8 8 ``` | [Flag](https://app.codecov.io/gh/brentyi/tyro/pull/122/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/122/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Brent+Yi) | `99.58% <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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