[RFC] Drop Python 3.6 from build matrix

[adamus1red]

Python 3.6 no longer receiving security fixes as of 2021-12 as per PEP-494.

[dgw]

Note that Sopel 8.0 will drop Python 3.6 soon (sopel-irc/sopel#2227), and has already dropped <=3.5 (sopel-irc/sopel#2062).

[adamus1red]

Fixed with #56