Add support for comments containing Unicode

[DevynCJohnson]

When using Flake8 (with Flake8-Mypy) on Python scripts containing comments with Unicode comments, Flake8 crashes with a UnicodeEncodeError. The suggested code in my commit appears to resolve the issue without side-effects.

[ambv]

I don't understand how the current behavior (which sets encoding to 'utf8' in the NamedTemporaryFile) is any worse than your suggested encoding of every line to utf8 and writing in binary.