Pin jinja2 for docs build

Some deprecated features were removed in jinja2==3.1.0.
For details see e.g. pallets/jinja#1626 and
readthedocs/readthedocs.org#9037
We should probably update sphinx instead, but that breaks other
things...

setup.cfg

@@ -70,6 +70,7 @@ doc =
     pyregion
     pyx>=0.15
     runnotebook
+    jinja2<3.1.0
     sphinx==3.1.2
     sphinx-bootstrap-theme
     sphinx-rtd-theme