python: determine Python (3.10) version number correctly. This change fixes https://bugs.gnu.org/44239 (and https://bugzilla.redhat.com/show_bug.cgi?id=1889732). * m4/python.m4: use print('%u.%u' % sys.version_info[:2]) for the version number instead of merely sys.version[:3], so the numbers are treated as numbers. * t/python-vars.sh (PYTHON_VERSION): Likewise. * doc/automake.texi: Document it. * NEWS: mention it. (Minor tweaks from Karl Berry.)