bpo-33238: Add InvalidStateError to concurrent.futures. (GH-7056)

Future.set_result and Future.set_exception now raise InvalidStateError
if the futures are not pending or running. This mirrors the behavior
of asyncio.Future, and prevents AssertionErrors in asyncio.wrap_future
when set_result is called multiple times.

Lib/concurrent/futures/__init__.py

@@ -10,6 +10,7 @@ from concurrent.futures._base import (FIRST_COMPLETED,
                                       ALL_COMPLETED,
                                       CancelledError,
                                       TimeoutError,
+                                      InvalidStateError,
                                       BrokenExecutor,
                                       Future,
                                       Executor,