Update phpunit/phpunit requirement from ^8.4 to ^9.0 (#114)

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-9.0.md)
- [Commits](sebastianbergmann/phpunit@8.4.0...9.0.0)

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

composer.json

@@ -25,7 +25,7 @@
         "php-http/guzzle6-adapter": "^2.0",
         "php-http/message": "^1.8",
         "php-http/mock-client": "^1.3",
-        "phpunit/phpunit": "^8.4",
+        "phpunit/phpunit": "^9.0",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "extra": {