Fix instance type of `app.config`

[jichengzhi]

I fixed the issue by overriding the class attribute declaration of quart.app.config_class.

Sorry that I'm confused with doc updates and don't know what to do. I think I do not need to update CHANGES.rst and .. versionchanged:: related sections.

• fixes #312

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] 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.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.

[aenglander]

Will this break backwards compatibility for those who have already used the workaround? Does it matter?