Make mkdir.py work under python3 versions below 3.5 (#8194)

* Make mkdir.py work unter python3 versions below 3.5
Because early versions of python3 did not have the optional arg "exist_ok" for "pathlib.Path(...).mkdir(...)" a build under this versions will abort with an error message.
This PR will modify the python script so that it works even under python 3.4 (and below).

tools/mkdir.py

@@ -12,7 +12,10 @@ def main():
     parser.add_argument('dir', action='store', nargs='+')
     ns = parser.parse_args()
     for p in ns.dir:
-        pathlib.Path(p).mkdir(parents=ns.parents, exist_ok=True)
+        try:
+            pathlib.Path(p).mkdir(parents=ns.parents)
+        except FileExistsError:
+            pass
     return 0
 
 if __name__ == '__main__':