Set known_{first,third}_party isort settings

setup.cfg

@@ -11,7 +11,10 @@ line_length=100
 multi_line_output=4
 skip=migrations,project_template
 add_imports=from __future__ import absolute_import,from __future__ import unicode_literals
+known_first_party=wagtail
 
+# These dependencies are frequently installed in "develop" mode which may cause isort to detect them as first party
+known_third_party=willow,modelcluster,taggit,django
 
 [pytest]
 django_find_project = false