Fix type error

Author: browniebroke

twine/commands/check.py

@@ -18,7 +18,7 @@
 import io
 import logging
 import re
-from typing import Dict, List, Optional, Tuple, cast
+from typing import Dict, List, Optional, Sequence, Tuple, cast
 
 import readme_renderer.rst
 from rich import print
@@ -111,7 +111,7 @@ def _check_file(
             )
 
     # Check classifiers
-    dist_classifiers = cast(Optional[List[str]], metadata["classifiers"])
+    dist_classifiers = cast(Sequence[str], metadata["classifiers"])
     for classifier in dist_classifiers:
         if classifier not in all_classifiers:
             errors.append(