Pin mistune to pre-2.x to fix doc build (#3285)

doc/requirements.txt

@@ -10,4 +10,7 @@ sphinx-panels
 furo
 # docutils 0.18.0 causes "AttributeError: 'Values' object
 # has no attribute 'section_self_link'" error when building doc
-docutils==0.17.*
+docutils==0.17.*
+# mistune 2.0.0-rc1 causes "AttributeError: module 'mistune'
+# has no attribute 'BlockGrammar'" error when building doc
+mistune<2.0.0