Small type hint and style issues

[jcrozum]

I've fixed a lot of the type hinting stuff so that we can be quite strict with our type checkers, but there are still a couple of issues to work out:

1. Type hinting and linting in the unit test files are not quite as rigorous as in the main library.
2. In space_utils.expression_to_space_list the types of the pyeda objects are still mysterious to me. I just ignore them during type checking for now.

[jcrozum]

We should also run everything through a linter eventually. There are lots of small PEP8 violations, like trailing whitespace and docstring formatting issues. A low priority, but something we should fix before full release.

[jcrozum]

I did this in #74, so it should all be more or less OK now. There are probably still a few issues with pyeda types and the auto-generated type stubs almost certainly have some errors, but things are passing strict linting and type checks now.