core: cleanup deprecations, exclude from type checking and show runtime warnings

among affected things:

- core.common.assert_never
- core.common.cproperty
- core.common.isoparse
- core.common.mcachew
- core.common.the
- core.common.tzdatetime
- core.compat.sqlite_backup

my/time/tz/main.py

@@ -2,10 +2,10 @@
 Timezone data provider, used to localize timezone-unaware timestamps for other modules
 '''
 from datetime import datetime
-from ...core.common import tzdatetime
+from my.core.common import datetime_aware
 
 # todo hmm, kwargs isn't mypy friendly.. but specifying types would require duplicating default args. uhoh
-def localize(dt: datetime, **kwargs) -> tzdatetime:
+def localize(dt: datetime, **kwargs) -> datetime_aware:
     # todo document patterns for combining multiple data sources
     # e.g. see https://github.com/karlicoss/HPI/issues/89#issuecomment-716495136
     from . import via_location as L