They are instead replaced by slightly roundabout ways to get hold of the
same objects (CompilerError, CCompiler).

Only distutils.sysconfig remains; we should investigate whether the
stdlib's sysconfig is a satisfactory replacement for it.

See #21057.

(Even if we waited for setuptools to correctly export these names or provide
replacement APIs, it would be slightly awkward to immediately require a very
new setuptools version for build.)

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).