Fix the pyflakes invocation in Nix so it checks our source files.

Fold `PassValidationTests._test_lease_operation_fails_without_passes` into its caller.

It turns our that `pyflakes` with no arguments expects input
on stdin. In CI, this is likely connected to `/dev/null` so it
passes.

Also fix the errors that pyflakes is reporting.

It was orginally factored out since there were both `add_lease` and `renew_lease methods`.
However, since tahoe 1.16, the client no longer defines `renew_lease`, and we removed it
from our protocol in #231. Given that there is now only one caller, we can remove the
factoring.

isodate duration parser has some bugs

Improve lease management for mutables

now client and server agree that only shares with associated writes in
`tw_vectors` are considered when computing cost

The remote method does not expose the renew_leases parameter, intentionally.

Previously an active lease on *any* share would cause us to disable lease
renewal entirely - including for newly written shares.

Now we will add a lease to any changed share that needs one, either because it
is growing or because it has no unexpired lease.