Issue #18999: Make multiprocessing use context objects.

This allows different parts of a program to use different methods for
starting processes without interfering with each other.

Lib/multiprocessing/heap.py

@@ -16,7 +16,7 @@ import tempfile
 import threading
 import _multiprocessing
 
-from . import popen
+from . import context
 from . import reduction
 from . import util
 
@@ -50,7 +50,7 @@ if sys.platform == 'win32':
             self._state = (self.size, self.name)
 
         def __getstate__(self):
-            popen.assert_spawning(self)
+            context.assert_spawning(self)
             return self._state
 
         def __setstate__(self, state):