chore: update quantecon-book-theme to v0.13.1 (#742)

- Fixes Colab launch button URL generation for flat notebook repositories
- Adds nb_path_to_notebooks configuration option
- See: https://github.com/QuantEcon/quantecon-book-theme/releases/tag/v0.13.1

Author: mmcky

environment.yml

@@ -7,7 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book==1.0.4post1
-    - quantecon-book-theme==0.13.0
+    - quantecon-book-theme==0.13.1
     - sphinx-tojupyter==0.4.0
     - sphinxext-rediraffe==0.2.7
     - sphinx-exercise==1.2.0