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/coding/commits.py

@@ -14,8 +14,7 @@ from typing import List, Optional, Iterator, Set, Sequence, cast
 
 
 from my.core import PathIsh, LazyLogger, make_config
-from my.core.cachew import cache_dir
-from my.core.common import mcachew
+from my.core.cachew import cache_dir, mcachew
 from my.core.warnings import high