Fix Python scripts for Windows: UTF-8 encoding (#485)

Fix Python scripts for Windows: UTF-8 encoding (#485)

To avoid problems with various symbols, we have to specify the encoding
when we read files.
The actual codec name is `utf_8` but aliases like `utf8`, `utf-8`, etc
are accepted. Here, I'm using `utf-8` alias.
https://docs.python.org/3.8/library/codecs.html#standard-encodings

This fixes `make lesson-check` when running under 'Git for Windows' for
lessons that have non-cp1252 characters.