build: Make looking for any Python version work as intended

* configure.ac: Don't look for Python 3.6 explicitly. Pass 3.6 as
minimum version when looking for any Python version. Fix the m4_if
condition comparing mym4pythonver to "any".

This fixes the fallback (previously using "all" and now using "any" as
version) which was intended to find any suitable Python version in case
looking for an explicit version didn't work. Now we only look for
Python versions that are still supported explicitly, but for the
fallback we allow any Python version >= 3.6 (which some LTS Linux
distros still ship).

• GnuPG-bug-id: T7262