Upgrade to python 3.9

[aplaice]

Anki now uses python 3.9 by default.

Our use of python 3.7 isn't in itself an issue (since python3 is mostly forwards-compatible).

However, given that we (or at least I :)) have pipenv issues when trying to update our dependencies with python3.7 (I'm not 100% sure what's causing them — a year ago the issue was that poetry newly used by some of our dependencies wasn't happy installing into a non-default directory (into dist/); the current issue with tabulate that I noticed is weird), while I don't have issues installing our dependencies with later python (other than dulwich, which will be fixed in the next dulwich release), I think upgrading to 3.9 should be worthwhile.

Also, if we want to "depend" (in terms of dev dependencies) on the latest Anki versions (rather than 2.1.26 like we do now), we need python 3.9, since the Anki package now requires python3.9.