Remove support for Python 3.6 and 3.7

[rogerbinns]

• [x] Update readme and index.rst
• [x] Update "versions" section of doc pointing to https://devguide.python.org/versions/ and give policy
• [x] Include list of which Python version was last supported by which apsw version
• [x] Update tests.py deletion of py 3.6 affected tests
• [x] Check all sys.version_info
• [x] Check dataclasses usage
• [x] Remove from megatest
• [x] Update Makefile
• [x] Update README.rst at release time