revert to fork multiprocessing

Author: aquadran

Lib/multiprocessing/context.py

@@ -320,12 +320,7 @@ def _check_available(self):
         'spawn': SpawnContext(),
         'forkserver': ForkServerContext(),
     }
-    if sys.platform == 'darwin':
-        # bpo-33725: running arbitrary code after fork() is no longer reliable
-        # on macOS since macOS 10.14 (Mojave). Use spawn by default instead.
-        _default_context = DefaultContext(_concrete_contexts['spawn'])
-    else:
-        _default_context = DefaultContext(_concrete_contexts['fork'])
+    _default_context = DefaultContext(_concrete_contexts['fork'])
 
 else: