Use PEP 484 / Mypy type annotations as part of CI

[SKalt]

Using Python3.6+ type annotations would require documenting and refactoring, as requested in #40

[SKalt]

I've got a mostly-done WIP at skalt/mypy-annotations. I could use some help on the lines marked FIXME or containing type Any. Here's a sample of the questions I've got:

• def names_and_types(cols: Dict[str, Any]) -> List[Tuple[str, Any]]:: What's c.dbtype?
• in def to_pytype(sqla_dialect: postgresql, typename: str), what's the type of the sqla_dialect.ischema_names.items()?
• Should InspectedSequence.__eq__ accept any instance of Inspected?
• What are the types of aruments to __init__? Unfotunatly, I've got this function for ... pretty much every class.