When you open a file in Python 3, it defaults to using the system charset to encode the file, which is typically UTF-8 on linux systems (good) but Windows-1251 on Windows (bad).
We need to add explicit encoding=utf-8 arguments to open() calls when we open CSV files for reading or writing. To complicate matters, the csv module works only with byte strings in Python 2, and only with unicode strings in Python 3, so we can't just blindly use `io.open` everywhere.