ValidationError with Swagger Petstore v3 specification

Hello,

I just started to play around with this library and took Open API Petstore v3 as my starting point. Now when trying to generate code I get:
`prance.ValidationError: Version mismatch: selected backend "flex" does not support specified version 3.0.2!`

Spec I'm using:
https://petstore3.swagger.io/ --> https://petstore3.swagger.io/api/v3/openapi.json

I also tried to validate spec by calling prance directly and got same error:

```
$ prance.exe validate petstore_v3.json
Processing "petstore_v3.json"...
-> Resolving external references.
ERROR in "petstore_v3.json" [ValidationError]: Version mismatch: selected backend "flex" does not support specified version 3.0.2!
```
But when changing backend to 'openapi-spec-validator' it works fine.
```
$ prance.exe validate --backend openapi-spec-validator petstore_v3.json
Processing "petstore_v3.json"...
-> Resolving external references.
Validates OK as OpenAPI 3.0.2!
```
-> prance seems to use flex as default backend if flex lib is installed.
--> So the question is: should roboswag library use 'openapi-spec-validator' as backend instead of 'flex' in case of v3 spec? Or am I missing something here?
See also: https://prance.readthedocs.io/en/latest/#compatibility

Reproduce the failure with the linked Petstore OpenAPI v3 specification using the prance validate entry point and compare the default flex backend with openapi-spec-validator. Trace how roboswag selects its validation backend; done means the intended backend behavior for OpenAPI 3.0.2 is established and covered by an appropriate regression check.

Written by the indexing model from the issue text.