Make Connection generic on Row

Make Connection generic on Row

We use a type variable 'RowConn' for Connection that is distinct from
'Row' that is used on Cursor side in order to reflect the possibility to
have distinct row factories on connection and cursor sides.

In order to avoid "propagation" of the Row type variable of Connection
classes, we use Any everywhere that variable is not used (namely,
everywhere outside the connection module).

The typing_example.py almost works: connect(row_factory=...) returns a
typed Connection, but only connect() still returns a Connection[Any].