typing: add py.typed to mercurial.cext for PEP 561 support Unfortunately, pytype doesn't support this yet. But it was included with the attr package, so we might as well do it here for consistency. Unlike the attr package, these type hints are only partial, so they are marked as such[1] (but who knows if it matters, given these are C extensions, so no local source code to scan). [1] https://peps.python.org/pep-0561/#partial-stub-packages