pyln: Add pytest to type ignores

Reported-by: Rusty Russell <@rustyrussell>

contrib/pyln-client/tests/test_plugin.py

@@ -1,7 +1,7 @@
 from pyln.client import Plugin
 from pyln.client.plugin import Request, Millisatoshi
 import itertools
-import pytest
+import pytest  # type: ignore
 
 
 def test_simple_methods():

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -5,7 +5,7 @@ from typing import Dict
 
 import logging
 import os
-import pytest
+import pytest  # type: ignore
 import re
 import shutil
 import sys