Refactor RequestSchema and ResponseSchema into a single Schema

[Sytten]

• RequestSchema is only needed in v2 to reject additional fields
• ResponseSchema is currently not playing too well with IDE (pycharm cough) because its missing a constructor with the kwargs (see ResponseSchema is missing constructor parameters #137)
• Having two to inherit from 2 schemas is not that nice since we use them a lot in requests and responses (see PR Improve "basics" docs #152)
• It's easy to forget and just use Schema from marshmallow (our doc does a poor job of recommending them too)

For those reasons I think we should refactor that into a single class that we use in all the tutorials and doc.

[airstandley]

I do like that having two classes makes the intention of derived classes clear. I hardly think that outweighs the negatives you've laid out though.

[airstandley]

Reading over the comments on #152, I'd like to propose that we name this unified single Schema something like StrictSchema.

I think that might make it a bit clearer when explaining that marshmallow.Schema works fine, but we recommend using StrictSchema to enforce the OpenApi specification on input/outputs.

[Sytten]

I like the name, I will work on the implementation