0.20.0

Bug fixes:

• Backwards-incompatible: Use HTTPStatus name rather than phrase
  to name error components. This fixes an issue due to phrase containing
  spaces not being URL-encoded. Also change DefaultError into
  DEFAULT_ERROR for consistency. This change will break code referencing
  one of those errors. (:issue:136).
  Thanks :user:michelle-avery for reporting.

Other changes:

• Backwards-incompatible: Remove OPENAPI_REDOC_VERSION and
  OPENAPI_SWAGGER_UI_VERSION. Remove hardcoded CDNs. Users should modify
  their code to use OPENAPI_REDOC_URL and OPENAPI_SWAGGER_UI_URL
  instead. The docs provide examples of CDN URLs. (:issue:134).