2and3/mmap.pyi : remove "type: ignore" (#425)

First was for mypy compatibility, but is no longer needed.

Second is removed by going inheriting (Iterable[bytes],
Container[bytes], Sized, Reversible[bytes]) instead of
Sequence[bytes]. Should presumably catch more errors when using __getattr__

tests/pytype_blacklist.txt

@@ -10,5 +10,4 @@
 2and3/argparse.pyi
 2and3/logging/handlers.pyi
 2and3/logging/__init__.pyi
-2and3/mmap.pyi
 2and3/webbrowser.pyi