Add a typing example (mypy) test

Add a typing example (mypy) test

This example is checked by running mypy on it with both Python and C
implementation.

By having cursor and record values' type declared before assignment, we
make sure that expected types are consistent (otherwise mypy would raise
"Incompatible types in assignment" errors).

Example check_row_factory_connection() is still incomplete because
Connection is not generic on Row; it thus contain many Any types.