cext: make revlog.c PY_SSIZE_T_CLEAN
Without this, Python 3.8 emits a deprecation warning, as using
int for # values is deprecated. Many existing modules use
PY_SSIZE_T_CLEAN, so this shouldn't be contentious.
I audited the file for all # formatters and verified we are
using Py_ssize_t everywhere now.
Differential Revision: https://phab.mercurial-scm.org/D6196
tests: add optional output for Python 2.7 deprecation
We already had one of these a few lines above. We need it here as
well.
Differential Revision: https://phab.mercurial-scm.org/D6203