Remove Python 2 from docs

[hegner]

BEGINRELEASENOTES

• Remove mention of Python 2 compatibility
  ENDRELEASENOTES

[tmadlener]

Should we enforce a minimum python version also in our cmake config? Currently we require that it is the same as the one that has been used for building root, but that doesn't strictly guarantee > 3.6:

podio/CMakeLists.txt

Line 82 in 2d36032

SET(REQUIRE_PYTHON_VERSION ${ROOT_PYTHON_VERSION})

Additionally, we could us the Python::Interpreter target (link) in the generator macro instead of python to avoid running into problems with systems that still have a python that is python2.