Adjust Windows-only logic in __init__.py

[ndgrigorian]

This PR suggests small changes to Windows-specific logic in __init__.py

hasattr(os, "add_dll_directory") is unnecessary after platform check and is removed, and sys.platform == "win32" is used which is more commonly seen in the ecosystem and avoids importing platform module

• Have you provided a meaningful PR description?
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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2553/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.19.0dev3=py313h509198e_6 ran successfully.
Passed: 1227
Failed: 0
Skipped: 9

[coveralls]

coverage: 72.107% (-0.001%) from 72.108%
when pulling 445ca81 on adjust-windows-platform-check
into 77a90bf on master.

[antonwolfy]

Thank you @ndgrigorian

[antonwolfy]

Could you please add the PR to the changelog also?

[ndgrigorian]

Could you please add the PR to the changelog also?

done, it's been added