Update quantecon-book-theme to 0.19.0 and remove myst-nb pin (#826)

Author: mmcky

environment.yml

@@ -7,8 +7,7 @@ dependencies:
   - pip
   - pip:
     - jupyter-book>=1.0.4post1,<2.0
-    - myst-nb>=1.3.0,<1.4.0
-    - quantecon-book-theme==0.18.0
+    - quantecon-book-theme==0.19.0
     - sphinx-tojupyter==0.6.0
     - sphinxext-rediraffe==0.3.0
     - sphinx-exercise==1.2.1