Bump proptest from 1.3.1 to 1.4.0

Bumps [proptest](https://github.com/proptest-rs/proptest) from 1.3.1 to 1.4.0.
- [Release notes](https://github.com/proptest-rs/proptest/releases)
- [Changelog](https://github.com/proptest-rs/proptest/blob/master/CHANGELOG.md)
- [Commits](proptest-rs/proptest@v1.3.1...v1.4.0)

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

Signed-off-by: dependabot[bot] <support@github.com>