* Cleanup unused imports (F401) of mostly standard Python modules,
  or some internal but unlikely referenced modules
* Where internal imports are potentially used, mark with noqa
* Avoid redefinition of imports (F811)

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

There are places in distutils where we accept a single string from the user, and interpret it as a set of command line arguments.
Previously, these were passed on as a string unmodified to exec_command, and interpreted by subprocess in a platform-specific way.

Recent changes to distutils now pass a list of arguments to subprocess, meaning we have to split the strings ourselves.
While `shlex.split` is perfect on posix systems, it is not a good approximation of either the old or the expected behavior on windows.

Provides the building blocks needed to fix gh-12979