Bump httpcore from 0.12.3 to 0.16.3

Bumps [httpcore](https://github.com/encode/httpcore) from 0.12.3 to 0.16.3.
- [Release notes](https://github.com/encode/httpcore/releases)
- [Changelog](https://github.com/encode/httpcore/blob/master/CHANGELOG.md)
- [Commits](https://github.com/encode/httpcore/compare/0.12.3...0.16.3)

---
updated-dependencies:
- dependency-name: httpcore
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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