Issue #15646: Prevent equivalent of a fork bomb when using multiprocessing

on Windows without the "if __name__ == '__main__'" idiom.

Lib/multiprocessing/forking.py

@@ -336,7 +336,7 @@ else:
         '''
         Returns prefix of command line used for spawning a child process
         '''
-        if process.current_process()._identity==() and is_forking(sys.argv):
+        if getattr(process.current_process(), '_inheriting', False):
             raise RuntimeError('''
             Attempt to start a new process before the current process
             has finished its bootstrapping phase.