Give Option.show_default higher priority wrt Context.show_default

[janluke]

Fixes #1963.

After this, a non-null show_default passed to an option overrides the corresponding context setting.

Implementation notes

• This also fixes the previous type hint of Option.show_default, adding str to the Union.
• I didn't include str in the type of Context.show_default because, as far as I understand, passing a str makes sense for dynamic options (not for a global default).
• Added a static constant Option.DEFAULT_SHOW_DEFAULT = False. If you change your mind about the default, the tests I added don't require any change.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[janluke]

@davidism I think Option.DEFAULT_SHOW_DEFAULT is redundant. We could just set Context.show_default = False rather than to None. Let me know what you think.

EDIT: nope, this would break the propagation from parent context.

[janluke]

@davidism Give me a shout when it's time to release 8.1.0 and I'll resolve conflicts.