Bump phpunit/phpunit from 7.2.3 to 7.3.1

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.2.3 to 7.3.1.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-6.5.md)
- [Commits](sebastianbergmann/phpunit@7.2.3...7.3.1)

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