Add the python version since the functionality is deprecated,
and raise a couple of deprecation warnings in a few places.

Theses functions are marked as deprecated in the documentation, but
especially in existing codebase, programmers tends to not re-check
whether functions are deprecated. So trigger the warning when possible.

It's also more probable that a developer will drop deprecated
functionality if we immediately give them information about
replacement API, and not have them to go find it in the documentation.

Include deprecation information in DocString as well as many tools pull
documentation from there and not from docs.python.org.

Add test making sure `find_loader()` and `find_module()` Both emit a
deprecation warning.