Some third-party providers provide deb packages, which is nice of
them, but they often refer to Python as python, even when their
programs support python3.  It's messy of course, but that's why
the python-is-python3 package exists.

Since the third-party providers are providing a deb package, they tend
to refer to python rather than python3 in the Depends field,
  Depends: python
even though their software is compatible with python3.

To allow for this, the python-is-python3 package needs to add
  Provides: python

At the moment python-is-python3 only provides a /usr/bin/python
symlink.  To be useful for these third-party deb packages,
python-is-python3 also needs to add Provides: python
(versioned >= 3 if necessary).

No, the package is called python3 now, and won't change. There should be no
dependencies on python, these should be fixed by the third parties. I'm unaware
of any 'python' package in the deb packaging ecosystem.