Added 7.0 of phpunit for PHP 8 support

Author: Tyler King

composer.json

@@ -16,7 +16,7 @@
     "caseyamcl/guzzle_retry_middleware": "^2.3"
   },
   "require-dev": {
-    "phpunit/phpunit": "^6.2",
+    "phpunit/phpunit": "^6.2|^9.3",
     "phpdocumentor/phpdocumentor": "2.*",
     "squizlabs/php_codesniffer": "^3.0"
   },