chore(mypy): Add typings for cli/freeze

Author: tony

src/tmuxp/cli/freeze.py

@@ -171,12 +171,14 @@ def extract_workspace_format(
 
         workspace_format = extract_workspace_format(dest)
         if not is_valid_ext(workspace_format):
-            workspace_format = prompt_choices(
+            _workspace_format = prompt_choices(
                 "Couldn't ascertain one of [%s] from file name. Convert to"
                 % ", ".join(valid_workspace_formats),
-                choices=valid_workspace_formats,
+                choices=t.cast(t.List[str], valid_workspace_formats),
                 default="yaml",
             )
+            assert is_valid_ext(_workspace_format)
+            workspace_format = _workspace_format
 
     if workspace_format == "yaml":
         workspace = configparser.dump(