Use tool:pytest in setup.cfg to fix warning message when running pytest

setup.cfg

@@ -13,7 +13,7 @@ skip=migrations,project_template
 known_first_party=wagtail
 known_third_party=willow,modelcluster,taggit,django,unidecode,bs4,pytz,PIL
 
-[pytest]
+[tool:pytest]
 django_find_project = false
 python_files=test_*.py
 testpaths=wagtail