This is a small bug fix and compatibility update. It caps the versions

of Python-Markdown on Python 3 and fixes running rbext on Python 3.10.
It alo fixes a bug with switching between Source/Rendered tabs in the

Markdown review UI and fixes a bug that could manifest in very new

installations where comments of different types with the same database

IDs on the same review request would all appear to resolve or reopen

when toggling the issue state for one of them.

Built the docs. Checked for spelling errors, build errors, and bad links.