Work around Triage (pytz) failing to parse TZ+x timezone strings

See issue [2342](https://code.google.com/p/ankidroid/issues/detail?id=2342) and the code comment for more details.
Example bug report: http://ankidroid-triage.appspot.com/view_crash?crash_id=4751692675416064 (I didn't report this yet because that was my secret bug to reproduce a crash, lol)