New 3.1-based work on rich text
This pull request replaces merge request !408 (closed), due to two main reasons:
- It was based on the 3.0 branch, not 3.1
- It stored rich text as low level XML Elements. While this worked for creating new content, it wasn't very user-friendly for analyzing existing rich text cells.
At @charlie.clark request, discussions of this feature are now on https://groups.google.com/g/openpyxl-users/c/fV3GnRjvbm8
--HG-- branch : 3.1