[3.13] Support the "pager" binary in _pyrepl (GH-122878) (#124242)

Support the "pager" binary in _pyrepl (GH-122878)

Debian (and derivatives) provide a /usr/bin/pager binary, managed by the
alternatives system, that always points to an available pager utility.
Allow _pyrepl to use it, to follow system policy.

This is a very trivial change, from a patch that Debian has been
carrying since 2.7 era. Seems appropriate to upstream.
https://bugs.debian.org/799555
(cherry picked from commit 426569eb8ca1edaa68026aa2bab6b8d1c9105f93)

Co-authored-by: Stefano Rivera <stefano@rivera.za.net>
Co-authored-by: T. Wouters <thomas@python.org>