TL;DR:

In this repo:

$ git clone [email protected]:llvm/llvm-project.git /tmp/llvm-project 
$ cp filter-llvm.sh /tmp/llvm-project
$ pushd llvm-project 
$ bash filter-llvm.sh
$ popd
$ git remote add upstream /tmp/llvm-project
$ git pull upstream main
$ git remote add origin https://github.com/makslevental/python_bindings_fork
$ git push -u origin main -f

In the other repo:

$ git remote add upstream [email protected]:makslevental/python_bindings_fork.git
$ git subtree add -P $WHEREVER_YOU_WANT upstream main

Note, you will have to have at least one commit for this to work in the "other repo"; do git commit --allow-empty -m 'Initial commit' if not.

Then to pull changes from "this" repo:

$ git subtree pull -P $WHEREVER_YOU_WANT upstream main

Requires

https://github.com/newren/git-filter-repo