multiprocessing: Fix timeout args of AsyncResult methods (#1984)

The timeout argument of wait and get methods of AsyncResult accepts also
None so fix the type specification to include Optional.

Accepting None is not an implementation detail as it's clearly
documented too:

https://docs.python.org/2/library/multiprocessing.html#multiprocessing.pool.AsyncResult

https://docs.python.org/3/library/multiprocessing.html#multiprocessing.pool.AsyncResult

stdlib/2/multiprocessing/pool.pyi

@@ -8,8 +8,8 @@ from typing import (
 _T = TypeVar('_T', bound='Pool')
 
 class AsyncResult():
-    def get(self, timeout: float = ...) -> Any: ...
-    def wait(self, timeout: float = ...) -> None: ...
+    def get(self, timeout: Optional[float] = ...) -> Any: ...
+    def wait(self, timeout: Optional[float] = ...) -> None: ...
     def ready(self) -> bool: ...
     def successful(self) -> bool: ...
 

stdlib/3/multiprocessing/pool.pyi

@@ -10,8 +10,8 @@ from typing import (
 _T = TypeVar('_T', bound='Pool')
 
 class AsyncResult():
-    def get(self, timeout: float = ...) -> Any: ...
-    def wait(self, timeout: float = ...) -> None: ...
+    def get(self, timeout: Optional[float] = ...) -> Any: ...
+    def wait(self, timeout: Optional[float] = ...) -> None: ...
     def ready(self) -> bool: ...
     def successful(self) -> bool: ...