remove @staticmethod decorators

There might have been a time when calling a static method was faster
than a regular method, but that is no longer the case. According to
micro-benchmarks, it is 70% slower in CPython 3.13 and it also makes
executing the code of a class definition slower.

test/test_postprocessor.py

@@ -713,8 +713,7 @@ class MetadataTest(BasePostprocessorTest):
 }
 """)
 
-    @staticmethod
-    def _output(mock):
+    def _output(self, mock):
         return "".join(
             call[1][0]
             for call in mock.mock_calls