Update tensors.py

[Li-Xiang-Ideal]

Add LeviCivitaTensor

[rocky]

Pretty good adherence to myriad of little details we have in adding a new built-in function. Looks like you should run isort over this. If you install pre-commit then these checks will be run at git commit time. And the funny symbols that are allowed in side a docstring have to be reduced until sometime in the future when we ditch this home-grown documentation system.

I suppose mentioning these things too is missing from the developer's guide.

A couple of other small things. Please move those Python import statements from inside functions to the top. Yeah, I find this counter-intuitive. In fact much of the Python style stuff I find counter-intuitive or wrong. But I have been beaten down by so many Python fascists who insist there is "only one way to write things in Python" that I have given up here.

Also, please consider adding an entry to CHANGES.rst.

But again, all of this is very small stuff. Thanks for doing this.

[Li-Xiang-Ideal]

First time to experience isort and black 😅 Hope it passes the checks this time.

[mmatera]

Very nice! Thank you very much! LGTM

[rocky]

Very nice! Thank you very much! LGTM

I agree! Thanks for sticking with this.