cext: remove inline rewriting of argv
This only worked on Python 2. And since we dropped support for Python 2,
we can drop support for this functionality.
Differential Revision: https://phab.mercurial-scm.org/D12233
cext: remove PY23()
Since we always run on Python 3 now, we no longer need this
macro to support Python 2. We refactor all users to just use
the 2nd argument.
Differential Revision: https://phab.mercurial-scm.org/D12232