core/time: more flexible support for resolving TZ abbreviation -> TZ ambiguities

addresses https://github.com/karlicoss/HPI/issues/103

for now via experimental time.tz.force_abbreviations config variable
not sure if this whole things is doomed to be resolved properly

my/core/time.py

@@ -1,20 +1,50 @@
 from functools import lru_cache
 from datetime import datetime, tzinfo
+from typing import Sequence
 
 import pytz # type: ignore
 
-# https://gist.github.com/edwardabraham/8680198
-tz_lookup = {
-    pytz.timezone(x).localize(datetime.now()).tzname(): pytz.timezone(x)
-    for x in pytz.all_timezones
-}
-tz_lookup['UTC'] = pytz.utc # ugh. otherwise it'z Zulu...
+
+def user_forced() -> Sequence[str]:
+    # conversion from abbreviations is always ambiguous
+    # https://stackoverflow.com/questions/36067621/python-all-possible-timezone-abbreviations-for-given-timezone-name-and-vise-ve
+    try:
+        from my.config import time as user_config
+        return user_config.tz.force_abbreviations # type: ignore[attr-defined]
+    except:
+        # todo log/apply policy
+        return []
 
 
-# TODO dammit, lru_cache interferes with mypy?
+@lru_cache(1)
+def _abbr_to_timezone_map():
+    # also force UTC to always correspond to utc
+    # this makes more sense than Zulu it ends up by default
+    timezones = pytz.all_timezones + ['UTC'] + list(user_forced())
+
+    res = {}
+    for tzname in timezones:
+        tz = pytz.timezone(tzname)
+        infos = getattr(tz, '_tzinfos', []) # not sure if can rely on attr always present?
+        for info in infos:
+            abbr = info[-1]
+            # todo could support this with a better error handling strategy?
+            # otz = res.get(abbr, tz)
+            # if otz != tz:
+            #     raise RuntimeError(abbr, tz, otz)
+            res[abbr] = tz
+        # ugh. also necessary, e.g. for Zulu?? why is it not in _tzinfos?
+        # note: somehow this is not the same as the tzname!
+        tzn = getattr(tz, '_tzname', None)
+        if tzn is not None:
+            res[tzn] = tz
+    return res
+
+
+# todo dammit, lru_cache interferes with mypy?
 @lru_cache(None)
 def abbr_to_timezone(abbr: str) -> tzinfo:
-    return tz_lookup[abbr]
+    return _abbr_to_timezone_map()[abbr]
 
 
 def zone_to_countrycode(zone: str) -> str:
@@ -30,3 +60,6 @@ def _zones_to_countrycode():
         for timezone in timezones:
             res[timezone] = countrycode
     return res
+
+
+# todo stuff here could be a bit more defensive? e.g. dependent on policy