myst-parser 5.x requires markdown-it-py ~=4.2, but the tree vendors 3.0.0
and taskcluster/requirements.in caps it below 4. That cap was added in
524c505b0b when the previous myst-parser 5.1.0 update was backed out,
and is marked temporary: it only exists to keep myst-parser 2.0.0
resolvable. Lift it so the sphinx 9 upgrade can proceed.
rich is the only real consumer and requires markdown-it-py>=2.2.0.
Neither taskcluster-taskgraph nor mozilla-taskgraph depends on it.
Differential Revision: https://phabricator.services.mozilla.com/D324270