Lint for print statements

flake8-print is a flake8 plugin that checks for `print()` statements in
Python files.

This should save us having to manually spot these when reviewing pull
requests.

The `--enable=T` flag needs to be set until this bug is fixed:
https://github.com/JBKahn/flake8-print/issues/27

scripts/run_tests.sh

@@ -28,7 +28,7 @@ if [[ -z "$VIRTUAL_ENV" ]] && [[ -d venv ]]; then
   source ./venv/bin/activate
 fi
 
-flake8 .
+flake8 --enable=T .
 display_result $? 1 "Code style check"
 
 npm test