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

Previously Hypothesis spent a long while putting storage indexes on the
storage servers and creating a list of nodes with non-overlapping storage
indexes and then asserting that, out of no relevant shares, none had expired
leases.

Now all storage servers in the test will have all of the relevant shares and
so all should be visited and updated.