Update exchange API to be capsule following new convention
This PR updates the exchange API to be capsule following the new convention. Note that the exchange API python recommendation is not yet updated on the array-api side and so far there hasn't yet been downstream implementation, so this change should be safe in generally.
cc @Kathryn-cat
RFC thread https://github.com/dmlc/dlpack/issues/179
cc @leofang @rgommers also updated python_spec.rst here
Array api spec PR https://github.com/data-apis/array-api/pull/984
Sorry for delay, will try to finish my review in a day or two!