Deps: Bump httpcore from 0.17.3 to 0.18.0 in /backport-pull-request (#…

…828)

Bumps [httpcore](https://github.com/encode/httpcore) from 0.17.3 to 0.18.0.
- [Release notes](https://github.com/encode/httpcore/releases)
- [Changelog](https://github.com/encode/httpcore/blob/master/CHANGELOG.md)
- [Commits](encode/httpcore@0.17.3...0.18.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>