add multiprocessing.Process.kill and close (#2027)

See https://docs.python.org/3.7/library/multiprocessing.html#multiprocessing.Process.kill.

Fixes #2022.

stdlib/3/multiprocessing/__init__.pyi

@@ -57,6 +57,9 @@ class Process():
     def start(self) -> None: ...
     def run(self) -> None: ...
     def terminate(self) -> None: ...
+    if sys.version_info >= (3, 7):
+        def kill(self) -> None: ...
+        def close(self) -> None: ...
     def is_alive(self) -> bool: ...
     def join(self, timeout: Optional[float] = ...) -> None: ...