Response codes must be in string format

[MissiaL]

At the moment it is incorrect format:
https://github.com/tfranzel/drf-spectacular/blob/master/tests/test_extend_schema.yml#L53
Must be like "200"

Details:
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#patterned-fields-1

[tfranzel]

actually i stumbled upon this before but the the schema validation never complained about it (which runs for every test). fyi if you did not see this yet: ./migrate.py spectacular --file schema.yaml --validate

I didn't notice this in the spec though:
This field MUST be enclosed in quotation marks (for example, "200") for compatibility between JSON and YAML.

easy fix. i'll do it right now.