I wondered why getsizeof fails for _sre.SRE_Pattern objects when it
succeeds for _socket.socket or struct.Struct.

It turns out that _sre.SRE_Pattern defines the tp_getattr slot, and this
prevents attribute lookup from searching the base class (object).
This is a pity, because the base implementation (which use tp_basicsize)
does the right thing for many objects.

So I borrowed some code from the __format__ method, and here is a patch.
Now classes are not handled specially any more; the distinction is
between old-style instances, and all other objects.

All tests pass, but I may have missed something important...