Fix further NumbaDeprecationWarnings: migrate from jit to njit

[connortann]

Original issue: shap#2909

PR #9 attempted to solve the issue by explicitly specifying nopython=False, but inadvertently introduced other Deprecation warnings as nopython mode is being removed entirely.

The recommendation from numba is to disable the object mode behaviour, by setting nopython=True or equivalently using njit.

[connortann]

On the previous PR, I didn't check that the warnings were fixed on python 3.10. Checking the logs of this PR, those warnings seem to be properly fixed now.

The only Numba warnings that remain relate to deprecated types; I opened a separate issue for that: #70.

[connortann]

@dsgibbons would you be able to review?

The numba warnings seem to be of relatively high interest judging by the traffic, so it would be fab to get this issue fixed fully.

[dsgibbons]

@connortann thank you for bringing the increased traffic to my attention. I read over https://numba.readthedocs.io/en/stable/reference/deprecation.html as I was not too familiar with njit. Your PR looks good and it doesn't seem to have broken any tests that were broken before.