general: make time.tz.via_location user config lazy, move tests to my.tests package

also gets rid of the problematic reset_modules thingie

my/tests/calendar.py

@@ -0,0 +1,9 @@
+from my.calendar.holidays import is_holiday
+
+from .shared_tz_config import config  # autoused fixture
+
+
+def test_is_holiday() -> None:
+    assert is_holiday('20190101')
+    assert not is_holiday('20180601')
+    assert is_holiday('20200906')  # national holiday in Bulgaria