ENH: allow python scalars in binary elementwise functions

[ev-br]

towards #96

[ev-br]

TODO:

• [ ] other functions from https://github.com/data-apis/array-api/issues/807
• [ ] type annotations: currently it's still add(x1: Array, x2: Array); do we want to update (and if yes, how? ArrayOrScalar or verbose explicit Array | bool | int | float | complex)
• [ ] do does the standard need updating?

What's here now is likely too simple because this way python scalars can upcast arrays. So depending on what ends up in the standard this might need a tweak for "weak scalars" in the NEP 50 sense.

[ev-br]

cross-ref https://github.com/data-apis/array-api/pull/862

[asmeurer]

We need to make sure this properly disallows cross-kind combinations that aren't supported. That will presumably match the text here https://data-apis.org/array-api/latest/API_specification/type_promotion.html#mixing-arrays-with-python-scalars (if https://github.com/data-apis/array-api/issues/841 is implemented, real -> complex will also be allowed).