Update guzzlehttp/psr7 requirement from ^1.5 to ^1.6
Update guzzlehttp/psr7 requirement from ^1.5 to ^1.6

Updates the requirements on [guzzlehttp/psr7](https://github.com/guzzle/psr7) to permit the latest version.
- [Release notes](https://github.com/guzzle/psr7/releases)
- [Changelog](https://github.com/guzzle/psr7/blob/master/CHANGELOG.md)
- [Commits](https://github.com/guzzle/psr7/compare/1.5.0...1.6.1)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
slimphp/Slim