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 Cannot load QUART_ prefixed environment variables #312

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.

[aenglander]

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