Emphasize that people should not override __import__.

Emphasize that people should not override __import__.

With importlib getting used more and more, changing __import__ will
not work as well as people used to hope as it will potentially bypass
importers, etc. It also will not work with importlib.import_module()
as it uses "importlib.__import__" (i.e. importlib's implementation of
import) directly and not builtins.__import__.