Allow specifiying a timezone for connections

Hi there,
please implement setting the timezone on a connection level, similar to the way [mysql.connector works with `time_zone`](https://dev.mysql.com/doc/connector-python/en/connector-python-connectargs.html).

This allows ensuring a constant timezone like UTC independent from what the database might have configured.

This is not currently implemented in PyMySQL either: https://github.com/PyMySQL/PyMySQL/issues/624