contrib/python/*/Makefile: Fallback to unversioned 'python'

And pull these from the PATH by default.  This way systems like CentOS
that don't have a python3 can still execute 'make clean', which
doesn't care about the Python major version.

The setup.py shebang, mode change, and ./ prefixing helps address
cases where PYTHON is empty.  This could be the result of improper
user configuration:

  $ make PYTHON='' clean

It could also be the state on systems with no Python installed, in
which case you'll see:

  $ make PYTHON='' clean
  ./setup.py clean --all
  /usr/bin/env: 'python': No such file or directory
  make: *** [Makefile:13: clean] Error 127

I've also shifted the Python invocations to the end of the clean
recipies so that as much as possible gets cleaned up even on systems
without Python installed.

Signed-off-by: W. Trevor King <wking@tremily.us>

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