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/pdfs.py

@@ -15,8 +15,9 @@ from typing import NamedTuple, List, Optional, Iterator, Sequence
 
 
 from my.core import LazyLogger, get_files, Paths, PathIsh
+from my.core.cachew import mcachew
 from my.core.cfg import Attrs, make_config
-from my.core.common import mcachew, group_by_key
+from my.core.common import group_by_key
 from my.core.error import Res, split_errors