The previous changes for the 3.8 update are ready, but staging got
merged into master, so there are a few more challenges to tackle:
* Use python 3.10 now since it's actually supported and less effort to
build (3.9 isn't recursed into anymore).
* sphinx doesn't build with these overrides, so patch it out entirely
(i.e. drop `sphinxHook` where it's causing problems).
* backport a few jinja2 fixes for python 3.10 that were fixed in later
versions, but break because this env is stuck to 2.11.
918c22bd5f