Upgrade sphinx and theme dependencies

- Update quantecon-book-theme from 0.9.2 to 0.11.0
- Update sphinx-tojupyter from 0.3.1 to 0.6.0
- Update sphinx-exercise from 1.0.1 to 1.2.1

Author: mmcky

environment.yml

@@ -7,10 +7,10 @@ dependencies:
   - pip
   - pip:
     - jupyter-book==1.0.4post1
-    - quantecon-book-theme==0.9.2
-    - sphinx-tojupyter==0.3.1
+    - quantecon-book-theme==0.11.0
+    - sphinx-tojupyter==0.6.0
     - sphinxext-rediraffe==0.2.7
-    - sphinx-exercise==1.0.1
+    - sphinx-exercise==1.2.1
     - sphinxcontrib-youtube==1.4.1
     - sphinx-togglebutton==0.3.2