Type hints

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

まず create_ppol、Pool.acquire()、Connection.fetch()、Connection.fetchrow() の定義を読み、それからそれらの戻り値アノテーションを PEP 484 と比較します。完了条件は、列挙されたメソッドが、Pool、Connection、Record のリスト、および Record または None を含む正確な戻り値型を公開することです。