convert HTTPException.description to string

[davidism]

Allows HTTPDescription to be None again. Additionally allows it to be another object like a dict, which seems to a pattern that is used to pass structured information to turn into a JSON response. I'm not sure that's actually documented as supported, so I'm not changing the type annotations for now.

• fixes HTTPException.description can no longer be None in werkzeug 2.x. #2115

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.