PySequence_Check(), PyMapping_Check(): only return true if the

PySequence_Check(), PyMapping_Check(): only return true if the
corresponding "getitem" operation (sq_item or mp_subscript) is
implemented.  I realize that "sequence-ness" and "mapping-ness" are
poorly defined (and the tests may still be wrong for user-defined
instances, which always have both slots filled), but I believe that a
sequence that doesn't support its getitem operation should not be
considered a sequence.  All other operations are optional though.

For example, the ZODB BTree tests crashed because PySequence_Check()
returned true for a dictionary!  (In 2.2, the dictionary type has a
tp_as_sequence pointer, but the only field filled is sq_contains, so
you can write "if key in dict".)  With this fix, all standalone ZODB
tests succeed.