Stop assigning a version to our Validator

By giving our created Validator a version, we're also registering it as
one of the global jsonschema validators. From create()'s documentation:

    version (str) – an identifier for the version that this validator
    class will validate. If provided, the returned validator class will
    have its __name__ set to include the version, and also will have
    jsonschema.validators.validates automatically called for the given
    version.

Why wouldn't we want to do this?

First, this Validator effectively replaces the existing Draft4Validator
because they're both based on the "draft4" schema. Internally, that
means both will be assigned the 'http://json-schema.org/draft-04/schema'
meta-schema, and the second one to be created (ours) will replace the
first in the `jsonschema.validators.meta_schemas` dictionary.

I would have expected jsonschema to use the schema's 'id' property to
differentiate between them, but it doesn't appear to work that way, and
I don't know enough about JSON Schema to go further.

Second, some of the OAS30 validators assigned here assume that the
validator instance is of type OAS3Validator, and that's only true when
jsonschema.validate() is explicitly called with `cls=OAS30Validator`.

For example, required() expects the given validator to have `.read` and
`.write` attributes, and those are Validator extensions provide by the
OAS30Validator subclass.

This means that a generic call to jsonschema.validate() that attempts to
use one of these validators (because of the global registration caused
by the first issue above) will produce AttributeError exceptions:

    /usr/local/lib/python3.7/dist-packages/jsonschema/validators.py line 932 in validate
      error = exceptions.best_match(validator.iter_errors(instance))
    /usr/local/lib/python3.7/dist-packages/jsonschema/exceptions.py line 367 in best_match
      best = next(errors, None)
    /usr/local/lib/python3.7/dist-packages/jsonschema/validators.py line 328 in iter_errors
      for error in errors:
    /usr/local/lib/python3.7/dist-packages/openapi_schema_validator/_validators.py line 46 in required
      if validator.write and read_only or validator.read and write_only:
   AttributeError: 'Oas30Validator' object has no attribute 'write'

This AttributeError behavior could be improved by adding isinstance()
checks (or similar) to those validator function implementations, but I
think the greater issue is the fact that these validators are really
only meant to be used with OAS30Validator, and the global registration
pattern doesn't support a Validator subclass.

Hence, my proposed solution is to stop registering BaseOAS30Validator as
a global jsonschema validator, at least not until some of the underlying
behaviors described above can be addressed in openapi_schema_validator
or in jsonschema itself.

Author: jparise

openapi_schema_validator/validators.py

@@ -47,7 +47,6 @@
         u"deprecated": oas_validators.not_implemented,
     },
     type_checker=oas_types.oas30_type_checker,
-    version="oas30",
     id_of=lambda schema: schema.get(u"id", ""),
 )