MAINT: Update quantecon-book-theme==0.6.0 (#311)

* TST: Testing for new refactor of qe-book-theme

* Update to PypI version

Author: mmcky

environment.yml

@@ -9,7 +9,7 @@ dependencies:
   - pip:
     - jupyter-book==0.15.1
     - docutils==0.17.1
-    - quantecon-book-theme==0.4.3
+    - quantecon-book-theme==0.6.0
     - sphinx-tojupyter==0.3.0
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==0.4.1