fix type annotations for Url & MultiHostUrl build

[davidhewitt]

Change Summary

These methods have an incorrect type annotation - they return instances of Url and MultiHostUrl.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin

[davidhewitt]

please review

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@e3ceb4b). Click here to learn what that means.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #781   +/-   ##
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  Coverage        ?   93.67%           
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  Files           ?       99           
  Lines           ?    14290           
  Branches        ?       25           
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  Hits            ?    13386           
  Misses          ?      898           
  Partials        ?        6           

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[codspeed-hq]

CodSpeed Performance Report

Merging #781 will not alter performance

_{Comparing dh/fix-url-build-annotation (bf3c0de) with main (e3ceb4b)}

Summary

✅ 126 untouched benchmarks