Use tp_members in SpecificationBase and ClassProvidesBase; stop using private undoc API

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.

Remove checking of PURE_PYTHON at build time

Also add a line of missing whitespace.

And tox. It wasn't properly working to report coverage before.

Turns out that the C extensions build and work fine with PyPy, but don't use them by default. Let them
be forced, though.

Tests needed some refactoring to account for the various permutations.