Require concrete versions of dev dependencies

Hopefully, this will fix the dependency hell in PHP 7.3 and 7.4.

Author: bheisig

composer.json

@@ -33,9 +33,9 @@
         "j13k/yaml-lint": "^1.1",
         "phan/phan": "^2.4.6",
         "php-parallel-lint/php-parallel-lint": "^1.0",
-        "phpcompatibility/php-compatibility": "*",
+        "phpcompatibility/php-compatibility": "^9.3.5",
         "phploc/phploc": "^6 || ^5",
-        "phpmd/phpmd": "@stable",
+        "phpmd/phpmd": "^2.8.2",
         "phpstan/phpstan": "^0.12.7",
         "phpunit/phpunit": "^9 || ^8",
         "povils/phpmnd": "^2",
@@ -45,13 +45,12 @@
         "seld/jsonlint": "^1.7",
         "sensiolabs/security-checker": "^6.0.3",
         "sllh/composer-lint": "^1.0",
-        "squizlabs/php_codesniffer": "*",
+        "squizlabs/php_codesniffer": "^3.5.4",
         "symfony/dotenv": "^5.0.3"
     },
     "suggest": {
         "ext-xdebug": "Needed for code coverage with phpunit"
     },
-    "prefer-stable" : true,
     "autoload": {
         "psr-4": {
             "bheisig\\idoitapi\\": "src/"