cmake: Add support for PYTHON_DESIRED cmake param (e.g.: cmake -DPYTHON_DESIRED="3")
Experimental (and untested) support for Python3

Python unittest are still failing

There is probably a bug in gpgme in Python3 (https://bugs.launchpad.net/pygpgme/+bug/1212214)
Flask doesn't support Python3 yet.
tests: Fix python tests