Treat python shebangs without version specified (e.g. `/usr/local/bin/python'
and `/usr/bin/env python') as invalid, because ports always depend on specific
version of python (e.g. python2 or python3), and generic `python'
which is link to either of them may point to incorrect version or
be not available at all (since it's installed by separate optional
python metaport)

Approved by: portmgr (mat), python (sunpoet)
Differential Revision: D9332