Support the "pager" binary in _pyrepl (#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