Deprecate unused dpnp.config settings

[antonwolfy]

dpnp.config defines __DPNP_QUEUE_GPU__, __DPNP_OUTPUT_NUMPY__ and __DPNP_OUTPUT_DPCTL_DEFAULT_SHARED__, read from the DPNP_QUEUE_GPU, DPNP_OUTPUT_NUMPY and DPNP_OUTPUT_DPCTL_DEFAULT_SHARED environment variables, but nothing in dpnp uses them, so setting those variables has no effect.

This PR deprecates the three attributes before removing them: they are no longer module attributes but are served by a module-level __getattr__, which returns the same value as before (still read from the environment at import) and emits a DeprecationWarning:

`dpnp.config.__DPNP_OUTPUT_NUMPY__` is deprecated, and will be removed in a future release. It has no effect.

Any other unknown name still raises AttributeError. They can be removed in a future release by deleting _DEPRECATED_SETTINGS and __getattr__.

__DPNP_ORIGIN__ and __DPNP_RAISE_EXCEPION_ON_NUMPY_FALLBACK__ are still used and are not changed. The test suite's own use of the DPNP_QUEUE_GPU environment variable, which dpnp/tests/conftest.py reads directly to pick the GPU skip list, is not affected.

The new dpnp/tests/test_config.py checks that each deprecated attribute warns and returns its value, and that an unknown name raises AttributeError. Importing dpnp emits no warning, and the warning points at the caller's line.

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• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/3107/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev14=np2py314h8d9cdd5_6 ran successfully.
Passed: 1376
Failed: 0
Skipped: 6

[coveralls]

No base build to compare — deprecate-unused-config into master