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 '%'.

Lib/test/datetimetester.py

@@ -1351,6 +1351,17 @@ class TestDate(HarmlessMixedComparison, unittest.TestCase):
         #check that this standard extension works
         t.strftime("%f")
 
+    def test_strftime_trailing_percent(self):
+        # bpo-35066: make sure trailing '%' doesn't cause
+        # datetime's strftime to complain
+        t = self.theclass(2005, 3, 2)
+        try:
+            _time.strftime('%')
+        except ValueError:
+            self.skipTest('time module does not support trailing %')
+        self.assertEqual(t.strftime('%'), '%')
+        self.assertEqual(t.strftime("m:%m d:%d y:%y %"), "m:03 d:02 y:05 %")
+
     def test_format(self):
         dt = self.theclass(2007, 9, 10)
         self.assertEqual(dt.__format__(''), str(dt))