specify submodule branch

.gitmodules

 [submodule "jupytext"]
 	path = jupytext
 	url = https://github.com/mwouts/jupytext
+	branch = main

post-init.sh

 #!/usr/bin/env bash
 
 # Update notebooks in the demo directory from upstream repo
-git submodule foreach git pull
+git submodule foreach git pull origin main
 rm -r demo && cp -r jupytext/demo demo
 
 # Copy jupytext's jupyter config to open md and script files as notebooks by default.