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/via_location.py

@@ -17,8 +17,8 @@ from typing import Iterator, Optional, Tuple, Any, List, Iterable, Set, Dict
 
 import pytz
 
+from my.core.cachew import mcachew
 from my.core import make_logger, stat, Stats, datetime_aware
-from my.core.common import mcachew
 from my.core.source import import_source
 from my.core.warnings import high