fix: import cast for required const properties, since it's used in the template

Resolves openapi-generators#1150

Author: benweint

openapi_python_client/parser/properties/const.py

@@ -111,7 +111,7 @@ def get_imports(self, *, prefix: str) -> set[str]:
             back to the root of the generated client.
         """
         if self.required:
-            return {"from typing import Literal"}
+            return {"from typing import Literal, cast"}
         return {
             "from typing import Literal, Union, cast",
             f"from {prefix}types import UNSET, Unset",