Build Python 3.10 wheels

[mbachry]

Change Summary

Thanks for making a new release! I noticed, however, that Python 3.10 wheels are missing: https://pypi.org/project/pydantic/1.9.0a1/#files

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[samuelcolvin]

Good catch, thank you.