Incorporate downstream CEGID changes.

[amcgregor]

Care of CEGID, my current employer, there are a collection of downstream changes utilized by the RITA job offer multi-posting platform. This pull request aims to homogenize the changes, eliminate the need to pin specific branches instead of released versions, and generally improve the project.

• A large amount of Python 3 and modernization changes.
  • Version pins.
  • Typing.
• Dispatch protocol adaptions.
• Added a StatusHandlers extension to perform internal redirection based on response status code. E.g. "Friendly error pages", 503 "down for maintenance" presentation, &c.
• Additional test helpers.
• Improved test suite.