Report (in q/a) and fix (in shebangfix) python[23] shebangs

Currently, only python shebangs (e.g. /bin/python, /usr/local/bin/python,
/usr/bin/env python etc.) are reported by stage Q/A and fixed by
USES=shebangfix. We need to do the same for python[23] as well.

Before the problem was not noticeable since many ports had e.g.
USES=python:2, which added a dependency on python2 metaport, however
that's going to switch to USES=python:2.7, and neither it, nor more
widely used USES=python adds a dependency on metaports, so there's
very high probability that python[23] links are not available.

Approved by: portmgr (mat)
Differential Revision: https://reviews.freebsd.org/D13571