BUG: f2py script shebang may refer to wrong python

The f2py executable has a shebang which uses the default python, rather
than the python it was compiled for.  This causes issues for deployment
of numpy (+f2py) across systems which have different environments.

This fix uses sys.executable to determine the resulting hardcoded
python to use.
Merge pull request #3356 from cpelley/f2py_script

f2py executable script shebang