Clean up config files and unused test files

[antonwolfy]

This PR does a few small cleanups, one commit per change:

1. Duplicate pre-commit hook. check-case-conflict was listed twice in .pre-commit-config.yaml; the entry in the Git group is kept.
2. Workflow permissions. The job in bump-sycl-deps.yml grants contents: write and pull-requests: write without saying why; it now has a comment for each, like the other workflows. They are needed to push the branch with the updated pins and to open a pull request from it.
3. Duplicate flake8 entry. .flake8 listed dpnp/dpnp_algo/__init__.py: F401 twice in per-file-ignores.
4. Package dpnp/tests/third_party/__init__.py. The package_data globs covered tests/third_party/cupy/ and tests/third_party/cupyx/ but not the (empty) __init__.py of tests/third_party/ itself, so in an installed package dpnp.tests.third_party was an implicit namespace package, unlike in the source tree. The new tests/third_party/*.py glob adds only that file.
5. Remove the zero-copy tests in dpnp/tests/third_party/intel/. They import modules that no longer exist (e.g. dpctl.tensor.numpy_usm_shared), so all three were always skipped, and nothing else uses them. The __sycl_usm_array_interface__ interoperability they were meant to check is covered by the dpnp.tensor tests.
6. Remove dpnp/tests/third_party/numpy_ext/. It only called the tests_external runner.

pre-commit run --all-files passes with these changes.

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• 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/3109/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev14=np2py314h8d9cdd5_10 ran successfully.
Passed: 1376
Failed: 0
Skipped: 6

[coveralls]

No base build to compare — small-cleanups into master