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/browser/export.py

@@ -16,7 +16,7 @@ from my.core import (
     make_logger,
     stat,
 )
-from my.core.common import mcachew
+from my.core.cachew import mcachew
 
 from browserexport.merge import read_and_merge, Visit