Change subprocess constants to be type int (#1577)

This change modifies the PIPE, STDOUT, and DEVNULL constants in the
subprocess module to be of type 'int'.

Note that the Python 2 subprocess module was already typed this way;
this commit is just updating the Python 3 subprocess module in the same
way.

stdlib/3/subprocess.pyi

@@ -238,11 +238,10 @@ else:
                      ) -> Any: ...  # morally: -> _TXT
 
 
-# TODO types
-PIPE = ...  # type: Any
-STDOUT = ...  # type: Any
+PIPE = ...  # type: int
+STDOUT = ...  # type: int
 if sys.version_info >= (3, 3):
-    DEVNULL = ...  # type: Any
+    DEVNULL = ...  # type: int
     class SubprocessError(Exception): ...
     class TimeoutExpired(SubprocessError): ...