(with Tom)

Set up CI for Tahoe 1.16.x

Fix unicode/bytes handling in Tahoe config code

Merge remote-tracking branch 'origin/228.tahoe-lafs-1.16.x-ci' into 225.unicode-does-not-have-the-buffer-interface.2

Remove `renew_lease`, removed from Tahoe-LAFS 1.16, from our storage protocol

We can just have Hypothesis give us a POSIX timestamp and then twiddle our
clock with that value appropriately.  Not much less code but a lot less
explanation.

This is better than stuffing it in later.

Tahoe-LAFS 1.16 changes the way StorageServer determines the current time.
This is great because it gives us a nice place to plug in our fake time.  We
still have to support <1.16 which leaves some not-so-nice stuff around.

If RIStorageServer doesn't define it, don't try to wrap it.

Use `add_lease` instead of `renew_lease`

tahoe-lafs 1.15 depends on attrs<20
something else pulls in attrs 21.2.0
pip fails to resolve the dependencies correctly in this case