Merge branch '4.3' into 4.4

* 4.3:
  Do not extend the new SF 4.3 ControllerEvent so we can make it final
  Backported return type violation bugfixes.
  [FrameworkBundle] Fix BrowserKit assertions to make them compatible with Panther

Author: nicolas-grekas

composer.json

@@ -24,6 +24,9 @@
         "egulias/email-validator": "^2.0",
         "symfony/dependency-injection": "^3.4|^4.1|^5.0"
     },
+    "conflict": {
+        "symfony/mailer": "<4.4"
+    },
     "autoload": {
         "psr-4": { "Symfony\\Component\\Mime\\": "" },
         "exclude-from-classmap": [