Proper type hints for the states

Co-authored-by: pubpub-zz <[email protected]>

Author: alexey-v-paramonov

pypdf/_reader.py

@@ -647,7 +647,7 @@ def _build_field(
             if "/Off" not in retval[key]["/_States_"]:
                 retval[key][NameObject("/_States_")].append(NameObject("/Off"))
         elif obj.get(FA.FT, "") == "/Btn" and obj.get(FA.Ff, 0) & FA.FfBits.Radio != 0:
-            states = []
+            states: List[str] = []
             retval[key][NameObject("/_States_")] = ArrayObject(states)
             for k in obj.get(FA.Kids, {}):
                 k = k.get_object()