MAINT: Pin jinja2<3.1 to avoid breaking sphinx

Ref: sphinx-doc/sphinx#10291
Note: jinja2 is not a requirement that we need to directlys specify.

Author: has2k1

setup.cfg

@@ -61,6 +61,8 @@ doc =
     nbsphinx
     numpydoc>=0.9.1
     sphinx>=3.0.0
+    # Ref: https://github.com/sphinx-doc/sphinx/issues/10291
+    jinja2<3.1
 
 [options.package_data]
 plotnine = plotnine/tests/baseline_images/**/*.png, plotnine/data/*.csv