bpo-34735: Fix a memory leak in Modules/timemodule.c (GH-9418)

There was a missing PyMem_Free(format) in time_strftime().
(cherry picked from commit 91e6c871)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Misc/NEWS.d/next/Core and Builtins/2018-09-19-06-57-34.bpo-34735.-3mrSJ.rst

+Fix a memory leak in Modules/timemodule.c.  Patch by Zackery Spytz.

Modules/timemodule.c

@@ -776,6 +776,7 @@ time_strftime(PyObject *self, PyObject *args)
         if (outbuf[1] == L'y' && buf.tm_year < 0) {
             PyErr_SetString(PyExc_ValueError,
                             "format %y requires year >= 1900 on AIX");
+            PyMem_Free(format);
             return NULL;
         }
     }