Remove hardcoded python in tests

The tests hardcode 'python' as the interpreter. In general, this is a symlink to python2. However, when running `python3 setup.py test`, you expect tests to be run on python3. This removes the hardcoded python and uses the same interpreter with which `setup.py` was invoked. Of course, some of the partly-converted tests won't run on py3, so those are disabled (since the converted text is already compared against known-correct results, the running part is redundant, anyway).