Add missing slots to base classes

[ariebovenberg]

The effectiveness of __slots__ is reduced if they are missing in base classes. This was the case for a few classes in parso:

$ slotscheck parso
ERROR: 'parso.python.tree:Keyword' has slots but superclass does not.
ERROR: 'parso.python.tree:Lambda' has slots but superclass does not.
ERROR: 'parso.python.tree:Operator' has slots but superclass does not.
Oh no, found some problems!
Scanned 21 module(s), 136 class(es).

The fix appears easy. All tests seem to pass.

I discovered the slots issues with slotscheck, a tool I maintain. Of course, If you like, I can add it to CI as I've done for instagram/LibCST, sqlalchemy/sqlalchemy, and aio-libs/aiohttp.

[ariebovenberg]

Fixed flake8 issue

[davidhalter]

Thanks a lot @ariebovenberg!