Clean up a few Python 2 remnants (#8452)

CONTRIBUTING.md

@@ -435,8 +435,6 @@ Some further tips for good type hints:
 * avoid invariant collection types (`list`, `dict`) in argument
   positions, in favor of covariant types like `Mapping` or `Sequence`;
 * avoid union return types: https://github.com/python/mypy/issues/1693;
-* in Python 2, whenever possible, use `unicode` if that's the only
-  possible type, and `Text` if it can be either `unicode` or `bytes`;
 * use platform checks like `if sys.platform == 'win32'` to denote
   platform-dependent APIs;
 * use mypy error codes for mypy-specific `# type: ignore` annotations,