build: Remove support for Python 2
* configure.ac: Remove 2.7 from list of Python versions to look for. * m4/python.m4: Remove support for Python 2. Add future Python versions up to 3.20. Update some comments.
The list future Python versions and the updated comments were taken from
automake 1.17's python.m4 script.
- GnuPG-bug-id: T7262