Replace install behavior on import with direct invocation (now that 'enabled' logic is duplicated in pth file).

Update docstring to use imperative voice, provide a bit more context, and advise against copying of the behavior.

Consolidate distutils importing hacks into _distutils_importer package. Generate distutils-precedence.pth inline.

This reverts commit 30b883f0b8071a3b1472c884574f38ce0128e457.

This puts non-distutils imports first and removes one unused import.

With the new `.pth` file, these warnings are no longer necessary.

Programmatically disable coverage when running on PyPy.

Warn the user when distutils is present

Warn the user when distutils is present to discourage this usage and direct users to the recommended usage. Closes #2230.

Test Python 3.9-dev on GitHub Actions

Fix race in spawn

Allow spawn to accept environment. Avoid monkey-patching global state.

Allow spawn to accept environment. Avoid monkey-patching global state. Closes pypa/setuptools#2212 and closes pypa/distutils#5.

Change exec_module to load_module

Test Python 3.9-dev on Travis CI

Fixes #2246

Re-enable distutils adoption with escape hatch

Allow opt-in and opt-out of distutils adoption at run time with SETUPTOOLS_USE_DISTUTILS environment variable.