Drop six dependency

[m-aciek]

six is a Python 2/3 compatibility shim that's no longer needed in a Python 3-only codebase. Closes #121.

Changes

• apis.py: Replace six.with_metaclass(_JsonApiMetaclass, object) with native Python 3 metaclass syntax; drop from __future__ compat imports
• utils.py: Replace six.string_types with str; drop from __future__ compat imports; import abc from collections

# Before
class JsonApi(six.with_metaclass(_JsonApiMetaclass, object)):
    ...

def is_list(value):
    return isinstance(value, abc.Sequence) and not isinstance(value, six.string_types)

# After
class JsonApi(metaclass=_JsonApiMetaclass):
    ...

def is_list(value):
    return isinstance(value, abc.Sequence) and not isinstance(value, str)