Closes: #6710

ENH: Explain rebuilds

The Python docs for __import__ recommend using importlib.get_module().

https://docs.python.org/3/library/functions.html#__import__

> Note: This is an advanced function that is not needed in everyday
> Python programming, unlike importlib.import_module().

As importlib.get_module() uses the Python module cache and returns the
module, this also allows simplifying many module cache checks of use of
sys.modules.

importlib.get_module() has been available since Python 3.3.

Fix #6589: autodoc: Formatting issues with autodoc_typehints='none'