try to make mypy happy

src/psij/job_spec.py

@@ -153,7 +153,7 @@ def name(self, value: Optional[str]) -> None:
         self._name = value
 
     @property
-    def environment(self) -> Optional[Dict[str, Union[str, int]]]:
+    def environment(self) -> Optional[Dict[str, str]]:
         """Return the environment dict."""
         return self._environment