devel/py-swagger-spec-validator: Update to 3.0.4
- Simplify Makefile by switching to USE_PYTHON=pytest.
- Switch to the PEP517 build framework to make the port future-proof.
Changelog since 2.7.4:
https://github.com/Yelp/swagger_spec_validator/blob/v3.0.4/CHANGELOG.rst