Skip test_history_file if readline module is not available
The test_history_file
test will fail if import readline
fails. We're seeing this in !149 (merged) right now with the Python 3.4 built from source (see comments in that MR for ways to resolve that in the docker image).
Still, readline is an optional module, so the test suite shouldn't fail if it doesn't exist. I think it's okay to skip the test in that case.