Python 3 compatibility

[natebragg]

Done in stages, to help make clear the nature of each change.

I tested with:

python -m test.test_mini
python -m test.test_new_ast
python -m test.test_java
python -m test.test_java_precisely

Then I did the same with python3, and then compared the output.

The specific versions tested were 2.7.15rc1 and 3.6.7.

[natebragg]

With these last few changes, I have now also verified it against all the tests in test_axioms (or at least the ones that were passing before).