Bump isort requirement for testing and update configuration

setup.cfg

@@ -20,6 +20,8 @@ multi_line_output=4
 skip=migrations,project_template
 known_first_party=wagtail
 default_section=THIRDPARTY
+lines_between_types=1
+lines_after_imports=2
 
 [tool:pytest]
 django_find_project = false

setup.py

@@ -3,8 +3,9 @@
 from wagtail import __version__
 from wagtail.utils.setup import assets, check_bdist_egg, sdist
 
+
 try:
-    from setuptools import setup, find_packages
+    from setuptools import find_packages, setup
 except ImportError:
     from distutils.core import setup
 
@@ -55,7 +56,7 @@ testing_extras = [
     # For coverage and PEP8 linting
     'coverage>=3.7.0',
     'flake8>=3.6.0',
-    'isort==4.2.5',
+    'isort>=5.6.0,<6.0',
     'flake8-blind-except==0.1.1',
     'flake8-print==2.0.2',
     'doc8==0.8.1',

tox.ini

@@ -60,5 +60,5 @@ setenv =
 
 [testenv:flake8]
 basepython=python3.6
-deps=flake8>=2.2.0
+deps=flake8>=3.6.0
 commands=flake8