prepare the 2.3.5 maintenance release

doc/changelog.rst

 Changelog
 =========
 
-2.3.5 (XXXX-XX-XX)
+2.3.5 (2021-06-02)
 ------------------
 
- * n/a
+ * Fix deprecations for PHP 8 and new PHPUnit
 
 2.3.4 (2020-01-30)
 ------------------

src/Silex/Application.php

@@ -41,7 +41,7 @@ use Silex\Provider\HttpKernelServiceProvider;
  */
 class Application extends Container implements HttpKernelInterface, TerminableInterface
 {
-    const VERSION = '2.3.5-DEV';
+    const VERSION = '2.3.5';
 
     const EARLY_EVENT = 512;
     const LATE_EVENT = -512;