Bump mockttp from 3.8.0 to 3.9.1 (#214)

Bumps [mockttp](https://github.com/httptoolkit/mockttp) from 3.8.0 to 3.9.1.
- [Release notes](https://github.com/httptoolkit/mockttp/releases)
- [Commits](httptoolkit/mockttp@v3.8.0...v3.9.1)

---
updated-dependencies:
- dependency-name: mockttp
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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