Avoid exploding when PYTHONOPTIMIZE=2

This changes `pymysql/__init__.py` so that it only tries to set `Connect.__doc__` if `pymysql.connections.Connection.__init__.__doc__` is set to anything.

When PYTHONOPTIMIZE=2, `__doc__` is set to `None` instead of the docstring and the code currently raises a `TypeError`, preventing pymysql from being imported.

pymysql itself may not benefit from `PYTHONOPTIMIZE=2` but this behavior prevents pymysql from being used in any other project that wants to use `PYTHONOPTIMIZE=2`.