bpo-32277: Fix exception raised from chmod(..., follow_symlinks=False) (#4797)

Author: asottile

Misc/NEWS.d/next/Library/2017-12-11-09-53-14.bpo-32277.jkKiVC.rst

@@ -0,0 +1,3 @@
+Raise ``NotImplementedError`` instead of ``SystemError`` on platforms where
+``chmod(..., follow_symlinks=False)`` is not supported.  Patch by Anthony
+Sottile.

Modules/posixmodule.c

@@ -2827,6 +2827,7 @@ os_chmod_impl(PyObject *module, path_t *path, int mode, int dir_fd,
                                                    dir_fd, follow_symlinks);
             else
                 follow_symlinks_specified("chmod", follow_symlinks);
+            return NULL;
         }
         else
 #endif