tox: Use flake8-isort

tox.ini

@@ -36,12 +36,11 @@ deps = sphinx
 [testenv:flake8]
 skip_install = True
 commands =
-	flake8 {toxinidir} {posargs}
-	isort {toxinidir} -c
+	flake8 {toxinidir}
 deps =
 	flake8
+	flake8-isort
 	flake8-quotes
-	isort
 
 [coverage:run]
 source = oauth2_provider
@@ -54,9 +53,10 @@ application-import-names = oauth2_provider
 inline-quotes = double
 
 [isort]
-lines_after_imports = 2
+balanced_wrapping = True
+default_section = THIRDPARTY
 known_first_party = oauth2_provider
+line_length = 80
+lines_after_imports = 2
 multi_line_output = 5
 skip = oauth2_provider/migrations/, .tox/
-line_length = 80
-balanced_wrapping = True