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/lastfm.py

@@ -26,7 +26,8 @@ import json
 from pathlib import Path
 from typing import NamedTuple, Sequence, Iterable
 
-from my.core.common import mcachew, Json, get_files
+from my.core.cachew import mcachew
+from my.core.common import Json, get_files
 
 
 def inputs() -> Sequence[Path]: