Should not have negative DST offsets

In very few timezones pytz reports negative dst. This happens if a change to daylight saving time falls together with a reduction in GMT offset such that the last STD time gmt offset is higher than the DST gmt offset.
This happens for 'Europe/Kiev' and other TZ in that region and for 'Antarctica/Palmer' and 'America/Iqaluit'

I am suggesting the following fix tzfile.py

@@ -83,6 +83,14 @@
                     if not prev_inf[1]:
                         break
                 dst = inf[0] - prev_inf[0] # dst offset
+
+ if dst < 0: # negative dst? : try beyond
+ for j in range(i+1, len(transitions)):
+ stdinf = ttinfo[lindexes[j]]
+ if not stdinf[1]:
+ break # found std time
+ dst = inf[0] - stdinf[0] # dst offset
+
             tzname = inf[2]

             # Round utcoffset and dst to the nearest minute or the