Add missing method add_mutually_exclusive_group(...) to argparse group (#573)

stdlib/2and3/argparse.pyi

@@ -143,6 +143,7 @@ class _ArgumentGroup:
                      metavar: Union[str, Tuple[str, ...]] = ...,
                      dest: str = ...,
                      version: str = ...) -> None: ...
+    def add_mutually_exclusive_group(self, required: bool = ...) -> _MutuallyExclusiveGroup: ...
 
 class _MutuallyExclusiveGroup(_ArgumentGroup): ...