Fix datetime.astimezone() method (pythongh-83861)

WIP - test and pure python fix

Resolves pythongh-83861

Author: abalkin

Lib/datetime.py

@@ -1965,6 +1965,11 @@ def replace(self, year=None, month=None, day=None, hour=None,
     def _local_timezone(self):
         if self.tzinfo is None:
             ts = self._mktime()
+            # Detect gap
+            ts2 = self.replace(fold=1-self.fold)._mktime()
+            if ts2 != ts: # This happens in a gap or a fold
+                if (ts2 > ts) == self.fold:
+                    ts = ts2
         else:
             ts = (self - _EPOCH) // timedelta(seconds=1)
         localtm = _time.localtime(ts)

Lib/test/datetimetester.py

@@ -6212,6 +6212,10 @@ def test_system_transitions(self):
                     ts1 = dt.replace(fold=1).timestamp()
                     self.assertEqual(ts0, s0 + ss / 2)
                     self.assertEqual(ts1, s0 - ss / 2)
+                    # gh-83861
+                    utc0 = dt.astimezone(timezone.utc)
+                    utc1 = dt.replace(fold=1).astimezone(timezone.utc)
+                    self.assertEqual(utc0, utc1 + timedelta(0, ss))
         finally:
             if TZ is None:
                 del os.environ['TZ']