typing: add an assertion instead of blacklisting mercurial/extensions.py stable

typing: add an assertion instead of blacklisting mercurial/extensions.py

Differential Revision: https://phab.mercurial-scm.org/D10175

--- a/mercurial/extensions.py	Thu Mar 11 17:27:31 2021 -0500
+++ b/mercurial/extensions.py	Thu Mar 11 17:55:52 2021 -0500
@@ -912,6 +912,7 @@
     exts = {}
     for ename, ext in extensions():
         doc = gettext(ext.__doc__) or _(b'(no help text available)')
+        assert doc is not None  # help pytype
         if shortname:
             ename = ename.split(b'.')[-1]
         exts[ename] = doc.splitlines()[0].strip()