Search for python3 with a better list of names.

Right now we search with:

AC_CHECK_PROGS(PYTHON, [python python2 python2.7 python3 python3.3])

But we'd like to prefer python3, and we'd like to prefer more recent versions. Ben McGinnes, who reported this bug, recommends

AC_CHECK_PROGS(PYTHON, [python3.6 python3.5 python3.4 python3.3 python3 python2.7 python2])

As an alternative, he suggests that we have a look at the GPGME Python binding M4 files, modulo license issues.

Personally, I'm thinking we'd like something like "python3, then known python3.x versions in descending order, then python, then python2, then python 2.7".

Having typed this ticket out, "Python" no longer looks like a word to me.