python: Auto-check for all installed python versions.

* m4/python.m4 (AM_PATH_PYTHON): Add a 4th arg.
* configure.ac (available_languages): Remove separate python2 and
python3 and keep just python.  Simplify test for pythons.  Use an
explicit list of python versions to test.

This seems to be a starightforward chnage to support more than two
python versions. I am not sure why we had that complicated thing
before. On my box I get builds and run tests for 2.7, 3.4 and 3.5.
If 3.6, 3.7 or 3.8 are installed they should also work.

• GnuPG-bug-id: T3354
• Signed-off-by: Werner Koch <wk@gnupg.org>