refactor: add missing type hints to mysqlclient.connection

[kkirsche]

1. Set the default cursor as seen here: https://github.com/PyMySQL/mysqlclient/blob/dac24e7b05b83eb1511e25e3cd8f8c20b7fbe112/MySQLdb/connections.py#L54
2. Use BaseCursor for cursorclass to align with use / description: https://github.com/PyMySQL/mysqlclient/blob/dac24e7b05b83eb1511e25e3cd8f8c20b7fbe112/MySQLdb/connections.py#L85
3. Type the __exit__ method
4. type autocommit as Any because it forces the value to bool: https://github.com/PyMySQL/mysqlclient/blob/dac24e7b05b83eb1511e25e3cd8f8c20b7fbe112/MySQLdb/connections.py#L236
5. Type the cursor method to align with https://github.com/PyMySQL/mysqlclient/blob/dac24e7b05b83eb1511e25e3cd8f8c20b7fbe112/MySQLdb/connections.py#L241

[kkirsche]

Thank you for the feedback, integrated in 12e5e6c and 6c2eed2 👍

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉