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.

tox.ini

@@ -7,7 +7,9 @@ deps=
     py27: mock
 commands=python -m unittest discover -v tests
 [flake8]
+max-line-length = 99
 exclude=
     .git,
     dist,
-    docs
+    docs,
+    examples