[s-kovacevic]

Recently I started using typing in a flask application but since most of the stubs are generated, it didn't help much. I'm trying to improve stubs starting from what is used the most (view functions, render_template, jsonify etc).

Before I spend more time on this I'd like to get some feedback.

With this change, view functions are properly type checked, for example:

@core.route('/index', methods=['GET'])
def index() -> Union[str, Response]:
    content = os.getenv('content')
    if content is not None:
        return content
    return redirect('/bye')

Here is the reference that covers most of the changes for easier review:
https://github.com/pallets/flask/blob/master/flask/app.py#L1992

[JelleZijlstra]

Thanks for your contribution!