This suggests an integration test would be useful.

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.

They're all redundant on Python 3

and fix it

The later function takes a count of passes to generated, and passes
them to `make_passes`. This also changes `make_passes` to return `Pass`es,
which is what `get_passes` wants to receive.