python2_select: Create `python2` select group (https://trac.macports.org/ticket/34326)
Allows for creation of `python2`, `python2-config`, `pydoc2`, `idle2`, etc.,
improving adherence to PEP 394 (https://www.python.org/dev/peps/pep-0394).
git-svn-id: https://svn.macports.org/repository/macports/trunk/dports@136741 d073be05-634f-4543-b044-5fe20cf6d1d6