[Python] Mappings that implement `__arrow_c_schema__` never have their method invoked

[cpcloud]

Describe the bug, including details regarding any error messages, version, and platform.

The implementation of pyarrow.schema doesn't allow objects that are Mappings to take advantage of __arrow_c_schema__ because Mapping is checked before __arrow_c_schema__ is checked.

This seems like a bug considering that custom schema objects naturally fit into the Mapping concept.

Would it make sense to swap the order here and first check for __arrow_c_schema__ and then check whether an object is a mapping?

Component(s)

Python

[jorisvandenbossche]

Good point, thanks for the issue!
I don't think there was a specific reason for this order in the code (added in #37797, cc @wjones127). Will do a PR to switch the order.

[jorisvandenbossche]

-> #43486

[jorisvandenbossche]

Issue resolved by pull request 43486
#43486