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/local-docs-build.txt
 sphinx==3.5.2
 sphinx_rtd_theme==0.5.2
+docutils<0.18
 
 # Code tabs extension for GDScript/C#
 # Stay on 1.3.0 until https://github.com/readthedocs/readthedocs-sphinx-search/issues/82 is fixed.