* 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.

This is a follow-up to gh-7427. This can lead to hard to understand
build failures, and we've warned that this change was coming for two
releases.

See gh-7427