better define workable comparator

for some reason, simple-phpunit installs phpunit 9 with incompatible comparator 5.
this is probably a dependency mistake in phpunit. rather than declare a
global conflict that messes with other packages, we now require-dev the
versions that work.

composer.json

@@ -28,7 +28,8 @@
         "php-http/httplug": "^1.0 || ^2.0",
         "php-http/message-factory": "^1.0",
         "phpspec/phpspec": "^5.1 || ^6.1 || ^7.3",
-        "symfony/phpunit-bridge": "^6.4.4 || ^7.0.1"
+        "symfony/phpunit-bridge": "^6.4.4 || ^7.0.1",
+        "sebastian/comparator": "^3.0.5 || ^4.0.8"
     },
     "autoload": {
         "psr-4": {
@@ -56,7 +57,6 @@
     },
     "conflict": {
         "nyholm/psr7": "<1.0",
-        "sebastian/comparator": ">=5.0",
         "zendframework/zend-diactoros": "*"
     },
     "prefer-stable": true,