ci: resolve importlib-metadata errors

[shiftinv]

Summary

Both pdm and twine depend on importlib-metadata, but broke due to a change in importlib-metadata v8.0.0.
Since updating pdm is rather tricky, we just pin importlib-metadata<8 there for now:tm:.
In the case of twine, v5.1.1 no longer has this issue (and funnily enough, neither does 5.0.0, but 5.1.0 does).

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, ...)