Fix locate_file() type hints for older Pythons

🤦

src/pip/_internal/metadata/importlib/_dists.py

@@ -11,6 +11,7 @@
     Mapping,
     Optional,
     Sequence,
+    Union,
     cast,
 )
 
@@ -96,7 +97,7 @@ def read_text(self, filename: str) -> Optional[str]:
             raise UnsupportedWheel(error)
         return text
 
-    def locate_file(self, path: str | PathLike[str]) -> pathlib.Path:
+    def locate_file(self, path: Union[str, "PathLike[str]"]) -> pathlib.Path:
         # This method doesn't make sense for our in-memory wheel, but the API
         # requires us to define it.
         raise NotImplementedError