Devs: please run pylint and mypy using python 3.10 by default

[Edward K. Ream]

Leo will continue to support python 3.6 indefinitely, but imo Leo should use the latest testing tools.

Running mypy and pylint on python 3.10 by default solves compatibility problems with pylint and mypy options. I shall soon upgrade pylint's reference rc file, leo/test/pylint-leo-rc-ref.txt, so that it makes pylint as happy as possible with python 3.10.

This policy should be plenty safe enough. We can assume that pylint and mypy continue to improve :-) Any comments or questions?

Edward