Support additional (Python) types in more than the `type` validator

In issue #364 and the related PR the type checker was expanded to allow redefining how jsonschema identifies/validates a type, with the intention of allowing a named tuple to be used as a jsonschema "object" type. In practice, this does not appear to be enough, as other portions of jsonschema use the python "in" expression to test for or iterate over properties in an object. In a NamedTuple, this will actually test the values of the properties. Two specific examples (required and additionalProperties) highlight this:

from typing import NamedTuple
from jsonschema import Draft7Validator
from jsonschema.validators import extend
from jsonschema.exceptions import ValidationError

# dict or tuple w/ _fields is object (allowed NamedTuple as object)
def is_object_or_namedtuple(checker, instance):
    return isinstance(instance, dict) or (
        isinstance(instance, tuple) and hasattr(instance, '_fields'))

# redefine object to allow namedtuple
type_checker = Draft7Validator.TYPE_CHECKER.redefine(
    'object', is_object_or_namedtuple)

# extend validator with new type checker
Validator = extend(Draft7Validator, type_checker=type_checker)

# validator with foo as required property
validator_foo_required = Validator(schema={
    "type": "object",
    "properties": {
        "foo": {
            "type": "string"
        }
    },
    "required": ["foo"]
})

# validator with foo as optional property, no additional
validator_foo_optional_no_additional = Validator(schema={
    "type": "object",
    "properties": {
        "foo": {
            "type": "string"
        }
    },
    "additionalProperties": False
})

class Example(NamedTuple):
    foo: str


example = Example(foo="bar")

try:
    validator_foo_required.validate(example)
except ValidationError as e:
    print(f"Error: {e.message}")

try:
    validator_foo_optional_no_additional.validate(example)
except ValidationError as e:
    print(f"Error: {e.message}")

# the problem is that 'foo' is not "in" example..
print('Is foo in example? ', ('foo' in example))

# ... but 'bar' is
print('Is bar in example? ', ('bar' in example))

This is more of an FYI, as #364 was targeted at supporting NamedTuple, but it looks like there would be some bigger changes required to facilitate redefining how properties are identified/accessed in the NamedTuple.

Start by reading issue #364 and its related pull request, then reproduce the NamedTuple examples in this issue with the required and additionalProperties validators. Define how named-tuple properties should be identified and accessed consistently, and consider the work complete when both examples validate the intended properties without treating their values as property names.