Fixed debug view crash during autumn DST change.

This only happens if USE_TZ = False and pytz is installed (perhaps not
the most logical combination, but who am I to jugde?)

Refs #23714 which essentially fixed the same problem when USE_TZ = True.

Thanks Florian and Carl for insisting until I wrote a complete patch.

tests/utils_tests/test_dateformat.py

@@ -10,6 +10,11 @@ from django.utils.timezone import (
     get_default_timezone, get_fixed_timezone, make_aware, utc,
 )
 
+try:
+    import pytz
+except ImportError:
+    pytz = None
+
 
 @override_settings(TIME_ZONE='Europe/Copenhagen')
 class DateFormatTests(SimpleTestCase):
@@ -29,6 +34,18 @@ class DateFormatTests(SimpleTestCase):
         dt = datetime(2009, 5, 16, 5, 30, 30)
         self.assertEqual(datetime.fromtimestamp(int(format(dt, 'U'))), dt)
 
+    def test_naive_ambiguous_datetime(self):
+        # dt is ambiguous in Europe/Copenhagen. LocalTimezone guesses the
+        # offset (and gets it wrong 50% of the time) while pytz refuses the
+        # temptation to guess. In any case, this shouldn't crash.
+        dt = datetime(2015, 10, 25, 2, 30, 0)
+
+        # Try all formatters that involve self.timezone.
+        self.assertEqual(format(dt, 'I'), '0' if pytz is None else '')
+        self.assertEqual(format(dt, 'O'), '+0100' if pytz is None else '')
+        self.assertEqual(format(dt, 'T'), 'CET' if pytz is None else '')
+        self.assertEqual(format(dt, 'Z'), '3600' if pytz is None else '')
+
     @requires_tz_support
     def test_datetime_with_local_tzinfo(self):
         ltz = get_default_timezone()