my.calendar.holidays: unhardcode calendar, detect it from the location data

my/core/time.py

@@ -15,3 +15,18 @@ tz_lookup['UTC'] = pytz.utc # ugh. otherwise it'z Zulu...
 @lru_cache(None)
 def abbr_to_timezone(abbr: str) -> tzinfo:
     return tz_lookup[abbr]
+
+
+def zone_to_countrycode(zone: str) -> str:
+    # todo make optional?
+    return _zones_to_countrycode()[zone]
+
+
+@lru_cache(1)
+def _zones_to_countrycode():
+    # https://stackoverflow.com/a/13020785/706389
+    res = {}
+    for countrycode, timezones in pytz.country_timezones.items():
+        for timezone in timezones:
+            res[timezone] = countrycode
+    return res