Upgrade sphinx-tojupyter to 0.4.0 (#702)

Author: mmcky

environment.yml

@@ -8,7 +8,7 @@ dependencies:
   - pip:
     - jupyter-book==1.0.4post1
     - quantecon-book-theme==0.10.1
-    - sphinx-tojupyter==0.3.1
+    - sphinx-tojupyter==0.4.0
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==1.2.0
     - sphinx-proof==0.2.1