Detect swagger vs openapi and auto-select appropriate backend

[jayvdb]

Expected Behaviour

When flex is installed, it should not be the default unless the input document is swagger.

Minimal Example Spec

n/a

Actual Behaviour

When flex is installed, prance validate my_openapi3.yaml will emit

prance validate my_openapi3.yaml 
Processing "my_openapi3.yaml"...
 -> Resolving external references.
ERROR in "my_openapi3.yaml" [ValidationError]: ErrorDict(<class 'flex.exceptions.ErrorList'>, {'required': [ErrorDict(<class 'flex.exceptions.ErrorList'>, {'swagger': ['This value is required']})]})

The {'swagger': ['This value is required']} in there is reasonably informative if you know/remember what flex is looking for.
But it isnt a great UX for people using openapi3, which is going to be an increasing percentage of users as openapi v3 replaces swagger.

The easiest approach is to re-organise the imports in

prance/prance/util/__init__.py

Line 31 in bab4b38

def validation_backends():

, so that openapi_spec_validator is first, and IMO flex should be last it has seen releases recently at https://pypi.org/project/flex/, while swagger_spec_validator is better maintained.

If there is some backwards compatibility reason for keeping flex first, it should be possible for it to only be first if the context is a swagger input.

Steps to Reproduce

1. Install flex
2. Run prance validate any_openapi3_schema.yaml

Environment

• OS: Linux
• Python version: 3.10
• Swagger/OpenAPI version used: 3.0
• Backend: flex