Specify `UTF-8` encoding for every `open` function

[Andrei-Aksionov]

Hi there 👋

One might ask why do we need to specify encoding explicitly, when it should be by default utf-8. Well, not quite.

The default encoding is platform dependent (whatever locale.getpreferredencoding() returns)

Which means that on Windows we can have an error, and that is exactly what recently happened.

[rasbt]

This is a good point. I learned this the hard way when teaching and there were students with various Windows operating systems.