docs: adds typing to an example app

[lecovi]

Examples of typing for:

• String view
• JSON view
• Error Handler
• Before Request
• Class Based View

fixes #4574

[davidism]

Does mypy check this file when running the typing test env?

[lecovi]

This solves #4574

[lecovi]

@davidism Yes, It does! 👍🏽

[davidism]

I moved these from the examples to tests directory, and added the directory to the mypy config so that they are tested automatically. I also did some refactoring to organize additional tests in the future.

Based on your @app.route tests, I realized that the decorated functions weren't actually being type checked. I've adjusted the type definitions, so now a view function that is annotated with an invalid return value will actually fail to type check.

I reported two issues to mypy while working on this:

• Union as part of decorator return type is not accepted for decorated function python/mypy#12936
• cannot accurately check the type of a decorated function's parameter python/mypy#12945