Conditionalize more tests that require PyPI (#13699)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Use the existing `pypi` feature to conditionalize a number of tests that
attempt to access https://pypi.org and/or
https://files.pythonhosted.org. See
https://github.com/astral-sh/uv/issues/8970#issuecomment-2466794088.
There is no reason to believe that these are *all* of the tests that
need to be conditionalized on the `pypi` feature, but this should be a
solid step in the right direction.
## Test Plan
<!-- How was it tested? -->
This allows me to build and run the integration tests in [Fedora’s `uv`
package](https://src.fedoraproject.org/rpms/uv) without having to
manually skip tests that try to access PyPI. I confirmed that this
appears to accomplish that goal.
Otherwise, this should be tested by building and running the tests as
usual. As mentioned in
https://github.com/astral-sh/uv/issues/8970#issuecomment-2516181501, a
more complete solution would include CI tests that confirm these
features are working as intended. I’m not in a position to offer that.
