Vendoring is usually as simple as doing the following for me:
pip install -r requirements.txt -t src/vendor.
Adding src/vendor to sys.path, or maybe using a modified version of the snippet here where you don’t mess around with sys.modules/sys.path, and instead just use the object you get from importlib.util.module_from_spec to avoid conflicts. (I usually just append/insert to sys.path, and have not tested my suggestion yet)
It becomes a real pain when C extensions are involved. For this case, I imagine a native solution from Anki’s side can get quite complicated and will be probably fragile.
See this for an example of how I recently went about bundling non-pure python dependencies for multiple platforms and Anki versions.