Add optional docstrings to member descriptors. For backwards compatibility, this required all places where an array of "struct memberlist" structures was declared that is referenced from a type's tp_members slot to change the type of the structure to PyMemberDef; "struct memberlist" is now only used by old code that still calls PyMember_Get/Set. The code in PyObject_GenericGetAttr/SetAttr now calls the new APIs PyMember_GetOne/SetOne, which take a PyMemberDef argument. As examples, I added actual docstrings to the attributes of a few types: file, complex, instance method, super, and xxsubtype.spamlist. Also converted the symtable to new style getattr.