Backport of #11376.

PR #9998 introduced NpyIter_Close as part of the public C-API in order to resolve any WRITEBACK_IF_COPY semantics after iterator use. This PR removes it entirely, instead resolving writeback semantics in NpyIter_Deallocate.

Changes:

• Extend NpyIter_Deallocate to include writeback resolution if needed via the operand-specific flag
• Remove NpyIter_Close, adjust the c-api headers, documentation, and internal code.
• Extend nditer_deallocate which is called as tp_dealloc for the python np.nditer to check if writeback resolution has occurred (via a context manager __exit__ or a direct call to np.close), and warn appropriately. This part requires introspection into the NpyIter. I created a non-public function to do so, is there a better way?
• Remove a c-extension test for warnings if using NpyIter_Deallocate without NpyIter_Close
• Repurpose a c-extension test to pure python since the warning moved levels