configure: search also for rst2{man, html}.py

configure: search also for rst2{man, html}.py

On some systems the rst2{man,html} executables may have a slightly
different name, like rst2{man,html}.py.

Add this name variation to the Generic Programs check.

This specific variation is found on Gentoo Linux.

Cc: David Sommerseth <davids@openvpn.net>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210827144807.27004-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22777.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b61d1988582feb2e1ed1cf9e62700f484e0bc150)