Add the ability to redeem a voucher from a Ristretto-flavored PrivacyPass issuer.

Test with Nix on CI.

Just use `pip install` instead of `pip-compile` and `pip-sync`.

Probably better to just rely on Tahoe-LAFS to declare the correct Twisted
dependency rather than trying to duplicate it here.  Both solutions are
wrong.  This one is less work.

Several packaging fixes.

it didn't fail before when building tahoe-lafs directly, I don't know
why (something else pulls in setuptools but it's unclear what).

it does fail without this when running the system tests at integration time...

other approach fails because we cannot upgrade pytest to fix attrs
incompatiblity because pytest has dropped python 2.7 support in 5.x series

Newer typing in nixpkgs requires newer Hypothesis than it has.

come back and explain this if it does :/

Expose ZKAPAuthorizer in python27Packages