Modify children of BeautifulSoup object

A community contributor found a bug in translate-html where the Beautiful Soup API was being called incorrectly potentially leading to undefined behavior.

This fixes translation with beautifulsoup4>=4.12.1
Modifying contents list directly is not supported by BeautifulSoup:

If you want to modify a tag’s children, use the methods described in Modifying the tree. Don’t modify the the .contents list directly: that can lead to problems that are subtle and difficult to spot.