Add common request method with token refresh handling and caching

[tr4nt0r]

Adds a request method to be used by other API methods.

• Reduces duplicate code for exception handling, but exception messages have to be more generic
• Ensures the access token is valid.
  • If token is expired, the token will be refreshed before the actual request
  • If token is not expired it continues with the request
  • If the request fails with an authentication exception, is refreshed and the request retried
  • If we don't have a valid refresh token the token refresh method will raise an auth exception
• Support for cache headers
  • Responses that contain an etag will be cached in a dict for later re-use
  • When sending a request for an endpoint were an etag is available from a previous request, it will be sent in the headers
  • If the endpoint responds with a not modified status the response will be served from the cache
  • if the endpoint returns with a Ok status and contains en etag, the cache is updated before serving the response
  • If the API returns a not modified status but there is nothing cached, the request is retried without cache headers.

Currently only the load_lists method has been refactored to use the method. If this change is approved, I will proceed with the refactoring.