fix `%sql --close <sqlalchemy_connection_string>` error

[meihkv]

• This adds @classmethod, else sql.connection.Connection._close(args.close) from https://github.com/catherinedevlin/ipython-sql/blob/12f9742a0a4eded514cc95377c920330ccba5e72/src/sql/magic.py#L166 will get an error saying that descriptor argument is required. @classmethod will allow self to be passed through, and args.close will correctly pass as an argument for descriptor.

descriptor is a connection string.

• This removes cls.connections.pop(conn.name), else it will have a key error when attempting to pop an invalid key from the dictionary of connections. This line isn't necessary from what I can see. Perhaps someone else with a different use case may need this can chime in after testing.

[ken-approveshield]

For what it's worth, I'm also unable to close my connections as documented in the readme and get the same error, using a MySQL DB.

[catherinedevlin]

Thank you very much!