UPD: update quantecon-book-theme to v0.20.2 and enable language switcher for zh-cn#721

mmcky wants to merge 3 commits intomainQuantEcon/lecture-python-intro:mainfrom

update/theme-v0.20.2-language-switcherQuantEcon/lecture-python-intro:update/theme-v0.20.2-language-switcherCopy head branch name to clipboard