Add isort and flake8 to CI (#869)

* Run isort.

* Clean up unused assignments.

* Add flake8 linting step.

tests/unit/plugins/module_utils/acme/test_challenges.py

@@ -3,25 +3,25 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 from __future__ import absolute_import, division, print_function
+
+
 __metaclass__ = type
 
 
 import pytest
-
-from ansible_collections.community.internal_test_tools.tests.unit.compat.mock import MagicMock
-
-
 from ansible_collections.community.crypto.plugins.module_utils.acme.challenges import (
+    Authorization,
+    Challenge,
     combine_identifier,
     split_identifier,
-    Challenge,
-    Authorization,
 )
-
 from ansible_collections.community.crypto.plugins.module_utils.acme.errors import (
     ACMEProtocolException,
     ModuleFailException,
 )
+from ansible_collections.community.internal_test_tools.tests.unit.compat.mock import (
+    MagicMock,
+)
 
 
 def test_combine_identifier():