The python scripts invoked by 'make check' didn't work on python3
before. That was a problem on systems where 'python' is python3.
Fixes bug 11608; bugfix on 0.2.5.2-alpha.
Improvement on f308adf838, where we made the ntor
unit tests run everywhere... so long as a python curve25519 module
was installed. Now the unit tests don't require that module.