Python 3.9 support

See the commit message of the main patch. First commit is in !2 (closed).

All tests pass and I'm doing the usual "run it over large codebases" crash-testing with no failures yet.

I would like to land this for 5.6 as distros will need it soon, e.g. https://bugzilla.redhat.com/show_bug.cgi?id=1818007