Auto-refresh AuthCode flow token. (#435)

* Auto-refresh AuthCode flow token.

* Reformatted.

* Reformatted.

* Removed invalid syntax.

* Removed abstract class SpotifyAuthManager.

* Fix typo on docstrings.

* Optionally try to fetch main input parameters from environment.

Implements the capability of trying to fetch the following parameters from the environment, when they're not directly passed to the authorization handler.
The affected parameters are: client_id, client_secret, redirect_uri.
An SpotifyOauthError is raised if no value gets found.

* Removed f-string for Python2 compatibility.

* Fix line-too-long.

* Remove useless import.

* Add username to docstring.

* Remove redundant return.

* Fix empty lines print statement for backward compatibility with Python2.

* Update simple4 example.

* Set optional 'as_dict' parameter on OAuth 'get_access_token'.

* Update changelog.

Co-authored-by: Stéphane Bruckert <stephane.bruckert@gmail.com>

CHANGELOG.md

@@ -11,6 +11,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
  - Support for `playlist_cover_image`
  - Support `after` and `before` parameter in `current_user_recently_played`
  - CI for unit tests
+ - Automatic `token` refresh
+ - `auth_manager` and `oauth_manager` optional parameters added to `Spotify`'s init.
+ - Optional `username` parameter to be passed to SpotifyOAuth, to infer a `cache_path` automatically
+ - Optional `as_dict` parameter to control `SpotifyOAuth`'s `get_access_token` output type. However, this is going to be deprecated in the future, and the method will always return a token string
+
+### Changed
+  - Both `SpotifyClientCredentials` and `SpotifyOAuth` inherit from a common `SpotifyAuthBase` which handles common parameters and logics.
 
 ## [2.7.1] - 2020-01-20