fix(sku)!: change `SKU.subscriptions` to be a sync method

[shiftinv]

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Summary

SKU.subscriptions mistakenly returned a coroutine, unlike other iterator methods (which are sync methods returning an async iterator). This removes the async keyword.

https://docs.astral.sh/ruff/rules/unused-async/ might have caught this, but the rule is currently still in preview.

Checklist

• [x] If code changes were made, then they have been tested
  • [x] I have updated the documentation to reflect the changes
  • [x] I have formatted the code properly by running pdm lint
  • [x] I have type-checked the code by running pdm pyright
• [x] This PR fixes an issue
• [ ] This PR adds something new (e.g. new method or parameters)
• [x] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)