Reformat with black for Python 3.9

Depend on Tahoe-LAFS 1.17.1

This suggests an integration test would be useful.

Port to Python 3

TempDir.join(unicode) returns unicode
TempDir.join(bytes) raises an Exception
FilePath(unicode) produces a text-mode FilePath
StorageServer.__init__ raises an exception if passed bytes

So we can just rely on self.tempdir.path being the correct type already and
even if it weren't StorageServer would immediately tell us.

Co-authored-by: Tom Prince <tom.prince@private.storage>

However, we still cannot be frozen (because of Failure.cleanFailure) and
cannot use attrs exception support (because we want value-based comparison).