`niv update tahoe-lafs-master`

now required by tahoe-lafs-master

tahoe-lafs declares this with an environment marker so mach-nix no longer
detects it

Change the mach-nix provider default to wheel and drop lots of special cases

This also adds a work-around for zfec#50 (fixed, unreleased) which may not be
strictly necessary but I don't know when the next zfec will release will
happen and it doesn't hurt so let's go with it.

Also test against Tahoe-LAFS master on CI

Reformat with black for Python 3.9

Depend on Tahoe-LAFS 1.17.1

This suggests an integration test would be useful.