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/reddit/rexport.py

@@ -20,8 +20,8 @@ from my.core import (
     Paths,
     Stats,
 )
+from my.core.cachew import mcachew
 from my.core.cfg import make_config, Attrs
-from my.core.common import mcachew
 
 from my.config import reddit as uconfig