configure: Update python search order
Some systems don't ship with "python" by default anymore, only
"python3" or "python2" or some combination, so include those in the
configure search.
Discussion: https://www.postgresql.org/message-id/flat/1457.
1543184081%40sss.pgh.pa.us#
c9cc1199338fd6a257589c6dcea6cf8d