[lint] #162: Define classmethod

Found by pre-commit hook

tests/api/ws2p/test_ws2p.py

@@ -33,7 +33,8 @@ from duniterpy.documents.ws2p.messages import DocumentMessage
 
 
 class TestWs2p(unittest.TestCase):
-    def test_block(self):
+    @classmethod
+    def test_block(cls):
         json_sample = {
             "heads": [
                 {