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

[shiftinv]

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

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