Suppress distutils replacement when building or testing CPython.

[jaraco]

Fixes #2965. Fixes #3007.

[jaraco]

As I was exploring this issue, I first sought to detect both the presence of 'pybuilddir.txt' and whether invoked by 'setup.py', but that would only work for 3007 and not 2965. Tests can be invoked a number of ways, through '-m unittest discover', invocation of 'regrtest.py', invocation of '-m test', and by executing the test modules directly (and maybe more). So I've opted to rely on the less precise simple detection of 'pybuilddir.txt' in the cwd. This approach won't work if the build is executed not from the root of the repo, but that seems like a small concession to make to address the issue broadly.