Make importing asdfghjkl conditional to the installed optional dependency

[wiseodd]

@runame @aleximmer please review.

This should make pip install laplace-torch work with or without the old asdfghjkl dependency. To try this:

pip install -e .
pytest

pip install git+https://git@github.com/wiseodd/asdl@asdfghjkl
pytest

@runame if everything is good in this PR, you can directly upload it on PyPi.

Also, if you know a more elegant solution, please let me know.

[wiseodd]

Added both cases above to the CI.

[runame]

LGTM. Waiting for @aleximmer to approve it as well.