Conditionalize version_extras test on the pypi feature (#14536)
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## Summary
The `version_extras` test added in
85c0fc963b1d4f6c8130c4b4446d15b8f6ac8ac4 needs to connect to PyPI. This
PR conditionalizes it on the `pypi` extra so that people running the
tests offline don’t have to skip that test explicitly.
<!-- What's the purpose of the change? What does it do, and why? -->
## Test Plan
<!-- How was it tested? -->
I already ran `cargo test` in the git checkout to confirm I didn’t
somehow introduce a syntax error. I am also applying this PR as a patch
to [the `uv` package in Fedora](https://src.fedoraproject.org/rpms/uv),
which runs tests offline with the `pypi` feature disabled.
