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

mercurial/cext/py.typed

@@ -0,0 +1,1 b''
+partial