Pin docutils to <0.18 to fix CI due to new warning

requirements.txt

@@ -5,6 +5,7 @@
 # https://github.com/readthedocs/readthedocs.org/blob/master/requirements/docs.txt
 sphinx==4.0.2
 sphinx_rtd_theme==0.5.2
+docutils<0.18
 
 # Code tabs extension for GDScript/C#
 sphinx-tabs==3.2.0