[lint] #162: Define staticmethod

Found by pre-commit hook
Otherwise we get: R0201: Method could be a function (no-self-use)
https://stackoverflow.com/a/2674052

tests/api/ws2p/test_ws2p.py

@@ -31,7 +31,8 @@ from duniterpy.documents.ws2p.messages import DocumentMessage
 
 
 class TestWs2p(unittest.TestCase):
-    def test_block(self):
+    @staticmethod
+    def test_block():
         json_sample = {
             "heads": [
                 {