This PR adds class-based view support. In particular, it will support to use @input, @output, @doc, @auth_required decorator on view methods of the MethodView class. Here is an example:

from apiflask import APIFlask, Schema, input, output
from flask.views import MethodView

app = APIFlask(__name__)


class PetInSchema(Schema):
    pass


class PetOutSchema(Schema):
    pass


@app.route('/pets/<int:pet_id>')
class Pet(MethodView):

    decorators = [auth_required(auth), doc(responses=[404])]

    @output(PetOutSchema)
    def get(self, pet_id):
        pass

    @output({}, 204)
    def delete(self, pet_id):
        pass

    @input(PetInSchema)
    @output(PetOutSchema)
    def put(self, pet_id, data):
        pass

    @input(PetInSchema(partial=True))
    @output(PetOutSchema)
    def patch(self, pet_id, data):
        pass

To collect the OpenAPI doc info stored on view methods, I rewritten Flask's app.route() decorator. Thus, the MethodView class should be decorated with @app.route() instead of using app.add_url_rule().

• fixes Is there any plan for Class-Based-View? #2

Checklist:

• Add example application.
• 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 docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue and your username.
• Add **Version Changed** or **Version Added** note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.