Update changelog for 3.1.0-1 release

debian/changelog

+phpunit-complexity (3.1.0-1) experimental; urgency=medium
+
+  [ Sebastian Bergmann ]
+  * Implement ComplexityCollection::mergeWith()
+  * Implement Complexity::isFunction() and Complexity::isMethod()
+  * Implement ComplexityCollection::isFunction() and
+    ComplexityCollection::isMethod()
+  * Handle anonymous classes
+  * Prepare release
+
+ -- David Prévot <taffit@debian.org>  Thu, 28 Sep 2023 19:20:20 +0200
+
 phpunit-complexity (3.0.1-1) experimental; urgency=medium
 
   [ Sebastian Bergmann ]