v0.19.0

• Support several required fields during User creation
• Don't pass sigalg parameter when not signing login request
• ALLOW_SAML_HOSTNAMES validation for redirect
• Custom attribute mapping for Django user model (example)
• Slo absence workaround
• Metadata EntityID exception handling
• Fix unsigned authentication request to POST endpoint
• py38 Test fixes
• CI with Github actions
• Backend restructuring for easier subclassing
• Assertion consumer service now more extensible as a class-based view
  with hooks that can be overridden by subclass implementations.