OpenAPI 3.0 Validator allows byte as string type

[p1c2u]

Only string should be allowed.

Type Checker:
https://github.com/p1c2u/openapi-schema-validator/blob/master/openapi_schema_validator/_types.py#L13

Test:
https://github.com/p1c2u/openapi-schema-validator/blob/master/tests/integration/test_validators.py#L64

This will be breaking change

[p1c2u]

Fixed as #251