Fixes import of uvicorn server. (encode/uvicorn#2527)

[tback]

Summary

This fixes a wrong import as discussed in #2527. Impact should be minimal.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

Thanks.