Remove indirection in threading (issue #10968).

Remove indirection in threading (issue #10968).

The public names (Thread, Condition, etc.) used to be factory functions
returning instances of hidden classes (_Thread, _Condition, etc.),
because (if Guido recalls correctly) this code pre-dates the ability to
subclass extension types.

It is now possible to inherit from Thread and other classes, without
having to import the private underscored names like multiprocessing did.

A doc update will follow: a patch is under discussion on the issue.