gh-135228: Create __dict__ and __weakref__ descriptors for object (GH-136966)

gh-135228: Create __dict__ and __weakref__ descriptors for object (GH-136966)

This partially reverts #137047, keeping the tests for GC collectability of the
original class that dataclass adds `__slots__` to.
The reference leaks solved there are instead solved by having the `__dict__` &
`__weakref__` descriptors not tied to (and referencing) their class.

Instead, they're shared between all classes that need them (within
an interpreter).
The `__objclass__` ol the descriptors is set to `object`, since these
descriptors work with *any* object. (The appropriate checks were already
made in the get/set code, so the `__objclass__` check was redundant.)

The repr of these descriptors (and any others whose `__objclass__` is `object`)
now doesn't mention the objclass.

This change required adjustment of introspection code that checks
`__objclass__` to determine an object's “own” (i.e. not inherited) `__dict__`.
Third-party code that does similar introspection of the internals will also
need adjusting.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>