`asarray` is not strict enough

[lucascolley]

The spec requires asarray to support "a (possibly nested) sequence of Python scalars", but not nested sequences of arrays.

https://data-apis.org/array-api/draft/API_specification/generated/array_api.asarray.html

[ev-br]

This just came up in #70 (comment), motivated by scikit-learn adoption work.

AFAICS, it is indeed unspecified in the spec and inconsistent across array libraries. Thus:

• docs: clarify that behavior when provided a nested sequence to asarray is unspecified array-api#917 suggest to make it explicit in the spec
• A PR with a fix to array-api-strict is welcome; will need to check the fallout at least in scipy.