Revert "Make configure prefer python3 to plain python."
This reverts commit
f201da39edcd6ac1ab9a3edf3e20e2a73bbbe69e.
The buildfarm is not ready for python3, evidently, so we'll
give the owners some more time to get set up.
Discussion: https://postgr.es/m/
2872c9a0-4b0a-1354-d5f6-
94d6f85ba354@enterprisedb.com