Add back missing PartialMessageable import

[krittick]

Summary

This adds back the import for PartialMessageable which was inadvertently removed in #821.

Fixes #827

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• 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, typehinting, examples, ...)

[1oonie]

This is still imported under if TYPE_CHECKING by the way.

[krittick]

That does not expose the object for use outside of type checking. This needed to be added back.

[Dorukyum]

I think they mean there is a duplicate import and the one under type checking can be removed.

[krittick]

Ah, good catch then.