Fix Python SystemError for import after override

If a wrapped Python class was overridden (via the override class method), and then a wrapped subclass of that class was subsequently loaded from a different module, a SystemError would occur during module loading. This was because the subclass was setting its tp_base member to the override. For extension types, we have to subclass from the original class, not from the override.

See !8886 (merged) for the override implementation.

Closes #18594 (closed)

Backport: release