Fix CI (#13748)

It was failing:
https://github.com/python/mypy/actions/runs/3133286021/jobs/5086495727

Author: sobolevn

mypyc/doc/conf.py

@@ -36,7 +36,7 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = []  # type: ignore
+extensions = []  # type: ignore[var-annotated]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ["_templates"]