Use tp_members in SpecificationBase and ClassProvidesBase.

Instead of storing 1 and 2 attributes, respectively, in __dict__. This lets us eliminate inst_attr and its use of an undocumented API.

It also helps reign in the size of these objects. Before, on CPython 8, a ClassProvides object that subclasses both SpecificationBase and ClassProvidesBase required 408 bytes: 48 for the object, 360 for the dict.

Now it requires 72 + 232 = 304.

With a bit of work on defining __slots__ in Python, that can be brought much lower still, I think.

zope.security tests pass with this installed.

CHANGES.rst

@@ -11,6 +11,11 @@
   value forces the Python implementation to be used, ignoring the C
   extensions.
 
+- Change the C classes ``SpecificationBase`` and its subclass
+  ``ClassProvidesBase`` to store implementation attributes in their structures
+  instead of their instance dictionaries. This eliminates the use of
+  an undocumented private C API function.
+
 
 4.7.1 (2019-11-11)
 ==================