Update exchange API to be capsule following new convention

[tqchen]

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.

[tqchen]

cc @Kathryn-cat

[tqchen]

RFC thread https://github.com/dmlc/dlpack/issues/179

[tqchen]

cc @leofang @rgommers also updated python_spec.rst here

[tqchen]

Array api spec PR https://github.com/data-apis/array-api/pull/984

[leofang]

Sorry for delay, will try to finish my review in a day or two!