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/integration/test_non_user_endpoints.py

@@ -219,8 +219,7 @@ class AuthTestSpotipy(unittest.TestCase):
         with self.assertRaises(SpotifyException) as cm:
             self.spotify.user_playlist_create(
                 "spotify", "Best hits of the 90s")
-        self.assertTrue(cm.exception.http_status == 401 or
-                        cm.exception.http_status == 403)
+        self.assertTrue(cm.exception.http_status == 401 or cm.exception.http_status == 403)
 
     def test_custom_requests_session(self):
         sess = requests.Session()

tests/integration/test_user_endpoints.py

@@ -11,17 +11,17 @@ following environment variables
     'SPOTIPY_CLIENT_SECRET'
     'SPOTIPY_REDIRECT_URI'
 """
-
 from __future__ import print_function
 
+import os
+import sys
+
 from spotipy import (
     CLIENT_CREDS_ENV_VARS as CCEV,
     prompt_for_user_token,
     Spotify,
     SpotifyException,
 )
-import os
-import sys
 import unittest
 import warnings
 import requests