add flake8-print once bug on their end is fixed

[mattsb42-aws]

Triggered by #19, I want to add automated checking to make sure that errant print statements don't end up in code again. Towards that end, JBKahn/flake8-print looks like a good solution to this, despite its advertised "Alpha" staging.

Unfortunately, there is a bug with flake8-print that keeps it from working with flake8>=3.4.0 JBKahn/flake8-print#23.

Once that bug is fixed (looks like it should be soon), add this as a dependency in the flake8 tox testenv.

[mattsb42-aws]

Unblocked by JBKahn/flake8-print#6