Add isort and flake8 to CI (#869)

* Run isort.

* Clean up unused assignments.

* Add flake8 linting step.

plugins/modules/luks_device.py

@@ -3,7 +3,9 @@
 # GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-from __future__ import (absolute_import, division, print_function)
+from __future__ import absolute_import, division, print_function
+
+
 __metaclass__ = type
 
 
@@ -421,12 +423,12 @@ name:
 import os
 import re
 import stat
-
 from base64 import b64decode
 
 from ansible.module_utils.basic import AnsibleModule
 from ansible.module_utils.common.text.converters import to_bytes, to_native
 
+
 RETURN_CODE = 0
 STDOUT = 1
 STDERR = 2