tzical is not thread-safe

```
Traceback (most recent call last):
...
  File "dateutil/tz/tz.py", line 1109, in utcoffset
    return self._find_comp(dt).tzoffsetto
  File "dateutil/tz/tz.py", line 1062, in _find_comp
    return self._cachecomp[self._cachedate.index((dt, self._fold(dt)))]
IndexError: list index out of range
```

``_cachedate`` and ``_cachecomp`` are not protected by a lock.