fix behavior of show_default with context_settings

[dylanpilz]

Summary

Implemented changes requested in #115.
Closes #114, closes #115

Tasks

• Added unit tests
• Added documentation for new features (where applicable)
• Added release notes (using reno)
• Ran test suite and style checks and built documentation (tox)

Further details

Command.show_defaults overrides show_default, consistent with click version 8.1x

[dylanpilz]

@stephenfin Feel free to take a look at your earliest convenience. Would be great to finally put this to rest

[stephenfin]

Apologies for the delay on this. Can you rebase onto master, squash the commits and run black to fix the lint issues (pre-commit run -a will do the trick). Happy to merge once that's done.

[stephenfin]

Actually, I've just pulled this locally and merged it in. Thanks, @dylanpilz!