Correct editable metadata hook return type to str (#209)

`prepare_metadata_for_build_wheel` was initially typed to return
Optional[str] just like the editable metadata hook, but it got changed
to `str` during review. However, the editable hook's return type was
never corrected. This seems to be an oversight as I can't find anything
in the PEP 517 or 660 specifications allowing these hooks to return
None.

Co-authored-by: Pradyun Gedam <[email protected]>

Author: ichard26

src/pyproject_hooks/_impl.py

@@ -290,7 +290,7 @@ def prepare_metadata_for_build_editable(
         metadata_directory: str,
         config_settings: Optional[Mapping[str, Any]] = None,
         _allow_fallback: bool = True,
-    ) -> Optional[str]:
+    ) -> str:
         """Prepare a ``*.dist-info`` folder with metadata for this project.
 
         :param metadata_directory: The directory to write the metadata to