PIN: myst-nb>=1.3.0,<1.4.0 to fix dark mode code cells (#825)

myst-nb 1.4.0 (released 4 days ago) changed its CSS generation to use
@media (prefers-color-scheme: dark) for system dark mode detection.
Since this site does not set data-theme on the html element, users with
OS dark mode enabled see dark code cell backgrounds on the light-themed
site.

Pinning to myst-nb 1.3.0 restores the previous CSS behavior which only
activates dark mode via the explicit html[data-theme='dark'] selector.

Closes QuantEcon/quantecon-book-theme#372

Author: mmcky

environment.yml

@@ -7,6 +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
     - sphinx-tojupyter==0.6.0
     - sphinxext-rediraffe==0.3.0