Refinement of PKCE (#566)

* Improve SpotifyPKCE.get_authorization_url

Ensure that a code challenge has been generated for the auth url

* Clean up namespace in SpotifyPKCE._get_auth_response_interactive

* Duplicate parse_auth_response_url into SpotifyPKCE

* Update SpotifyPKCE security advisory

* Update changelog for PKCE refinements

CHANGELOG.md

@@ -7,7 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
-// Add your changes here and then delete this line
+### Added
+
+- `SpotifyPKCE.parse_auth_response_url`, mirroring that method in
+ `SpotifyOAuth`
+
+### Fixed
+
+- Using `SpotifyPKCE.get_authorization_url` will now generate a code
+ challenge if needed
 
 ## [2.14.0] - 2020-08-29