Revert "Actually use tomllib for reading metadata"

This reverts commit 08de8c3.

Author: srittau

lib/ts_utils/metadata.py

@@ -239,7 +239,12 @@ def read_metadata(distribution: str) -> StubMetadata:
     """
     try:
         with metadata_path(distribution).open("rb") as f:
-            data: dict[str, object] = tomllib.load(f)
+            # This cast is necessary for pyright to understand that the
+            # variable is a dict with object values. Just using
+            # `data: dict[str, object] = tomllib.load(f)` doesn't work because
+            # pyright still infers TOMLDocument which derives from
+            # dict[Unknown, Unknown].
+            data = cast(dict[str, object], tomlkit.load(f))
     except FileNotFoundError:
         raise NoSuchStubError(f"Typeshed has no stubs for {distribution!r}!") from None