Make PoolAcquireContext return a Connection type

[Yrlish]

__aenter__ in PoolAcquireContext should be defined to return a Connection type. This will help IDEs to properly help suggesting methods when using syntax:

with my_pool.acquire() as con:
  con  # this is now defined as type Connection by the acquire method
  pass

instead of doing this:

with my_pool.acquire() as con:  # type: Connection
  pass

Before:

After:

[Yrlish]

I forgot I have made an relevant issue 1.5 years ago #387.

[elprans]

There is a comprehensive effort to add type annotations in #577