bpo-35066: _dateime.datetime.strftime copies trailing '%' (GH-10692)

Previously, calling the strftime() method on a datetime object with a
trailing '%' in the format string would result in an exception. However,
this only occured when the datetime C module was being used; the python
implementation did not match this behavior. Datetime is now PEP-399
compliant, and will not throw an exception on a trailing '%'.

Modules/_datetimemodule.c

@@ -1528,10 +1528,13 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
             ntoappend = 1;
         }
         else if ((ch = *pin++) == '\0') {
-            /* There's a lone trailing %; doesn't make sense. */
-            PyErr_SetString(PyExc_ValueError, "strftime format "
-                            "ends with raw %");
-            goto Done;
+        /* Null byte follows %, copy only '%'. 
+         * 
+         * Back the pin up one char so that we catch the null check
+         * the next time through the loop.*/
+            pin--;
+            ptoappend = pin - 1;
+            ntoappend = 1;
         }
         /* A % has been seen and ch is the character after it. */
         else if (ch == 'z') {
@@ -1616,7 +1619,7 @@ wrap_strftime(PyObject *object, PyObject *format, PyObject *timetuple,
         usednew += ntoappend;
         assert(usednew <= totalnew);
     }  /* end while() */
-
+    
     if (_PyBytes_Resize(&newfmt, usednew) < 0)
         goto Done;
     {