chore: drop 'setuptools' dependency

[tseaver]

Closes #595.

[arithmetic1728]

I need the setuptools dependency for my mtls work (to add some C/C++ extension). Is there other way to solve the issue without dropping the dependency?

[tseaver]

@arithmetic1728

I need the setuptools dependency for my mtls work (to add some C/C++ extension). Is there other way to solve the issue without dropping the dependency?

install_requires (which is what this PR changes) is about runtime dependencies, not build-time. Given that you are going to script building the C extension and releasing binary wheels using e.g. Github Workflow actions (as in python-crc32c) or some other mechanism, you can guarantee that setuptools is present during the build without having to declare it in setup.py.

Users of google-auth shouldn't need to have setuptools present in order to install the wheels you build in CI.

[arithmetic1728]

@tseaver yep, you are right, thank you for the information!