Apischema

- Fix handling of generic dataclass merged fields
- Improve handling of generic types in json-schema generation

- Fix Python 3.9 support
- Fix PyPy3 support
- Add CI with Github Actions

- Fix default_schema not being called

- Improve Optional handling in schema generation
- Improve deserialization performance for dataclass primitive fields
- Add ValidationError deserializer
- settings.default_schema
- Pydantic compatibility example

- Fix openAPI definitions path
- Add default ref for other standard types or types with schema

- Update package description