Fix communicate to work with python3.
Fix communicate to work with python3.

I don't know why the tests didn't catch this. :-(

Auditors: colin

Test plan:
In webapp, ran
    python2 ../devtools/khan-linter/runlint.py flow-typed
    python3 ../devtools/khan-linter/runlint.py flow-typed
without error either time.

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter