Updates the requirements on [proptest](https://github.com/altsysrq/proptest) to permit the latest version.
- [Release notes](https://github.com/altsysrq/proptest/releases)
- [Changelog](https://github.com/AltSysrq/proptest/blob/master/CHANGELOG.md)
- [Commits](https://github.com/altsysrq/proptest/commits/0.8.6)

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