subtraction, work as documented.  In the Python implementation,
they weren't calling utcoffset() if both operands had the same
tzinfo object.  That's fine if it so happens that the shared
tzinfo object returns a fixed offset (independent of operand),
but can give wrong results if that's not so, and the latter
obtains in a tzinfo subclass instance trying to model both
standard and daylight times.  The C implementation was already
doing this "correctly", so we're just adding tests to verify it.