Not sure if this is a build system problem, or a test script problem. Autoconf will pick `python`, `python2`, `python3` in that order, so the scripts should be probably be written such that they run with either `python2` or `python3`, since systems where `python` is `python3` exist. `PYTHON=python2 ./configure` sidesteps the problem on systems where `python` is `python3`, but people shouldn't have to do that.