Optimize transition lookup for timezone handling in Python bindings.

This modifies the transition lookup for timezone handling classes to
start the search from the last used transition, rather than starting
from the beginning of the list each time.

mythtv/bindings/python/MythTV/utility/dt.py

@@ -24,15 +24,32 @@ class basetzinfo( _pytzinfo ):
                              'time utc local offset abbrev isdst')
 
     def _get_transition(self, dt=None):
+        try:
+            index = self.__last
+        except AttributeError:
+            index = 0
+
         if dt is None:
             dt = _pydatetime.now()
         dt = (dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second)
 
-        for i, transition in enumerate(self._transitions):
-            if dt < transition.local[0:5]:
-                transition = self._transitions[i-1]
-                break
-        return transition
+        direction = 0
+        while True:
+            if dt < self._transitions[index].local[0:5]:
+                if direction == 1:
+                    index -= 1
+                    break
+                else:
+                    direction = -1
+            else:
+                if direction == -1:
+                    break
+                else:
+                    direction = 1
+            index += direction
+
+        self.__last = index
+        return self._transitions[index]
 
     def utcoffset(self, dt=None):
         return timedelta(0, self._get_transition(dt).offset)