Add mp_context parameter to concurrent.futures.ProcessPoolExecutor (#3078)

Closes #3076

stdlib/3/concurrent/futures/_base.pyi

@@ -1,4 +1,4 @@
-from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Tuple, Optional, Set, NamedTuple
+from typing import TypeVar, Generic, Any, Iterable, Iterator, Callable, Tuple, Optional, Set
 from types import TracebackType
 import sys
 

stdlib/3/concurrent/futures/process.pyi

@@ -1,5 +1,5 @@
 from typing import Any, Callable, Optional, Tuple
-from ._base import Future, Executor
+from ._base import Executor
 import sys
 
 EXTRA_QUEUED_CALLS: Any
@@ -8,8 +8,11 @@ if sys.version_info >= (3,):
     class BrokenProcessPool(RuntimeError): ...
 
 if sys.version_info >= (3, 7):
+    from multiprocessing.context import BaseContext
+
     class ProcessPoolExecutor(Executor):
         def __init__(self, max_workers: Optional[int] = ...,
+                     mp_context: Optional[BaseContext] = ...,
                      initializer: Optional[Callable[..., None]] = ...,
                      initargs: Tuple[Any, ...] = ...) -> None: ...
 else:

stdlib/3/concurrent/futures/thread.pyi

@@ -1,5 +1,5 @@
 from typing import Any, Callable, Optional, Tuple
-from ._base import Executor, Future
+from ._base import Executor
 import sys
 
 class ThreadPoolExecutor(Executor):