extensions: avoid a crash when the version isn't properly byteified on py3
We already force bytestr on the testedwith and buglink attributes in
dispatch.py when generating a bug report with a similar comment about not
every extension being ported to py3. We should do the same here, so the
function can be properly typed.
Differential Revision: https://phab.mercurial-scm.org/D9433