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

tests/calendar.py

@@ -1,6 +1,19 @@
+from pathlib import Path
+
+import pytest # type: ignore
+
 from my.calendar.holidays import is_holiday
 
 
 def test() -> None:
     assert is_holiday('20190101')
     assert not is_holiday('20180601')
+    assert is_holiday('20200906') # national holiday in Bulgaria
+
+
+@pytest.fixture(autouse=True)
+def prepare(tmp_path: Path):
+    from . import tz
+    # todo meh. fixtures can't be called directly?
+    orig = tz.prepare.__wrapped__ # type: ignore
+    yield from orig(tmp_path)