Fix incorrect patchlevel information for 3.5.3+.

It got messed up when I merged my private 3.5.3 release
changes with new work happening in the 3.5 main branch.
(3.5 main branch was in "3.5.3rc1", my branch was "3.5.3+",
and Mercurial helpfully merged the two in a kind of nonsense way.)