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

首先阅读 create_ppol、Pool.acquire()、Connection.fetch() 和 Connection.fetchrow() 的定义，然后将它们的返回注解与 PEP 484 进行比较。完成标准是，所列方法公开准确的返回类型，包括 Pool、Connection、Record 列表以及 Record 或 None。