Restore Python 3.5 compatibility
Fixes
Traceback (most recent call last):
File "/home/mg/opt/python35/lib/python3.5/doctest.py", line 1321, in __run
compileflags, 1), test.globs)
File "<doctest undertime.guess_zone[5]>", line 1, in <module>
File "/home/mg/src/undertime/undertime.py", line 539, in guess_zone
return OffsetZone(zone)
File "/home/mg/src/undertime/undertime.py", line 342, in __init__
offset = int(match["offset"])
TypeError: '_sre.SRE_Match' object is not subscriptable