fix: optimize images (#1302)

Optimizing images of the Python course with
[optimizt](https://github.com/funbox/optimizt). The original images will
stay in the Git history forever, the damage has been done, but at least
we can improve the future experience of the readers of the docs.

Not sure if we could automate this, I guess the only way to do it would
be a sort of pre-commit check. I doubt there's a GitHub Action which
would optimize images and then automatically rebase the branch to get
rid of the old ones. Anyway, that's a challenge for a different day,
this is just a small PR with a simple and straightforward manual
improvement.

Author: honzajavorek