Add support for openapi-core 0.16

[damonhook]

Summary

Add support for openapi-core >= 0.16 (currently 0.16.0 and 0.16.1), which also adds support for openapi-spec-validator>=0.5.0 (refs #173).

Lib Changes

• Added support for openapi-core >= 0.16
  • Minimum supported version would now be openapi-core == 0.16.0 as there are a lot of breaking changes in the 0.15 and 0.16 releases.

Note There are some discussion points in this draft PR for how we handle some potentially breaking changes

Dev Changes

• Examples
  • Added extra endpoints to the todoapp example so that it covers more use cases
    • Query Parameters, Body Parameters, Path Parameters, Combinations, etc.
• Isort
  • Removed deprecated not_skip and -rc options from isort commands as these have been removed in the 5.0 release (https://pycqa.github.io/isort/docs/upgrade_guides/5.0.0.html)
  • Added black profile to isort to avoid conflicts between isort and black
• Flake8
  • Pin to < 5 for now as flake8-mock does not yet have support for flake8 >= 5

Note I am busy doing some tests using some of our larger applications at the company I work for