add back in sphinx-proof

Author: mmcky

environment.yml

@@ -11,6 +11,7 @@ dependencies:
     - sphinx-tojupyter==0.3.1
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==1.0.1
+    - sphinx-proof==0.2.1
     - sphinxcontrib-youtube==1.4.1
     - sphinx-togglebutton==0.3.2
     - sphinx-reredirects==0.1.4