Ensure pip is available when running the -no_extras tox factor

[kurtmckee]

I'm working to get the test suite passing locally -- it's been failing for multiple reasons, which has impaired my ability to thoroughly test other PRs I've submitted.

This PR addresses a missing pip dependency when using the tox-uv plugin; the tox environments are created without pip installed, which causes the -no_extras factors to fail. pip isn't added to the pip-compile-managed requirements files because pip-compile warns that "it is considered unsafe" to have pip listed in the requirements.

Fixed

• Ensure that pip is available when running the -no_extras tox factor.

[nedbat]

I don't understand how the tox-uv plugin is involved. Can you show me how to reproduce the problem?

[kurtmckee]

pipx install tox
pipx inject tox tox-uv

tox

The tox-uv plugin uses uv to create the virtual environments into which tox installs dependencies. However, uv doesn't automatically install pip.

I use the plugin because it helps my ailing old Windows laptop run tox test suites measurably faster, but it has a side-effect that usually doesn't affect test suites...unless they need to run pip. 😅

Adding pip as a listed requirement resolves the issue, and also makes the dependency explicit.