Type hints #387

Hi there!

I would really appreciate if the methods were defined with correct return data type as specified in PEP-484.

This would help with typing hints where the IDE can correctly determine how to help out with the right methods and variables.

Some examples on places I've stumbled into this:

• create_ppol should type hint Pool
• Pool.acquire() should type hint Connection
• Connection.fetch() should type hint list of Record
• Connection.fetchrow() should type hint Record or None