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

Improve Tahoe-LAFS packaging

They fail a lot so leave them disabled.

We do so by introducing a limitation that the client must be configured with
an issuer and it must match the issuer announced by any storage server the
client is to talk to.

The client will decline to speak to any storage server without a matching
issuer.

This may make no difference but we're not doing anything to override
openssl_1_1 in this overlay so we don't need to get it from `self`.

Although ... the client resource doesn't really the part of the redeemer that
needs the announcement.  Maybe that part of the redeemer belongs somewhere
else.  Probably.