[StableOnly] Fix docs job

This is a combination of multiple commits from master:

  60a7b11 docs: pin mistune version due to breakage
  7544f8f requirements: Bump doc requirements
  0968130 Revert "docs: prevent build error by rolling back Sphinx version"
  65547c8 docs: prevent build error by rolling back Sphinx version
  bdb049c requirements: Update doc requirements

Though we don't usually change requirements on stable branches, this one
seems okay since it only affects docs builds and not production code.

Signed-off-by: Stephen Finucane <stephen@that.guru>

docs/requirements.txt

@@ -1,5 +1,7 @@
-sphinx>=2.0
-reno>=2.2
-sphinx_rtd_theme~=0.5.0
-jinja2~=2.11.2
+sphinx>=4.2.0
+reno>=3.4.0
+sphinx-rtd-theme~=1.0
+jinja2~=3.0
 sphinxcontrib-openapi~=0.7.0
+# See https://github.com/getpatchwork/patchwork/issues/442
+mistune<2.0.0