Support the "pager" binary in _pyrepl

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

Lib/_pyrepl/pager.py

@@ -36,6 +36,8 @@ def get_pager() -> Pager:
         return plain_pager
     if sys.platform == 'win32':
         return lambda text, title='': tempfile_pager(plain(text), 'more <')
+    if hasattr(os, 'system') and os.system('(pager) 2>/dev/null') == 0:
+        return lambda text, title='': pipe_pager(text, 'pager', title)
     if hasattr(os, 'system') and os.system('(less) 2>/dev/null') == 0:
         return lambda text, title='': pipe_pager(text, 'less', title)