cext: make parsers.c PY_SSIZE_T_CLEAN
This is needed to avoid a deprecation warning in Python 3.8. I believe
the conversion of int to Py_ssize_t is harmless in the changed
locations. But this being C code, it should be audited with care.
Differential Revision: https://phab.mercurial-scm.org/D6197
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors