Linting of core subpackage + clean up imports (#437)

* Linting of OAuth2 mod. + update tests to detect core package path

* Tweak imports in tests + update tox ini to ignore Flake8 E501 error

* Tweak tox ini - ignore examples

* Remove `f`-string from OAuth2 mod.

* More import tweaks in core package + tests

* Update flake8 config. in tox ini - set line length limit to 99 chars.

tests/unit/test_oauth.py

@@ -1,11 +1,11 @@
 # -*- coding: utf-8 -*-
-
-import six.moves.urllib.parse as urllibparse
-from spotipy import SpotifyOAuth
 import io
 import json
 import unittest
 
+import six.moves.urllib.parse as urllibparse
+
+from spotipy import SpotifyOAuth
 
 try:
     import unittest.mock as mock