Exception: Callable has no attribute __abc_registry

Expected Behavior

Cybersource-REST-Client-Python should work with the Python 3.7.

Actual Behavior

Users receive a Callable has no attribute __abc_registry error.

This is a known issue when installing typing on a system that has Python > 3.6 because typing is in the standard library since version 3.5. Importing typing with Python 3.7 fails.

Cloudwatch Log

[ERROR] AttributeError: type object 'Callable' has no attribute '_abc_registry'
Traceback (most recent call last):
  File "/var/lang/lib/python3.7/imp.py", line 234, in load_module
    return load_source(name, filename, file)
  File "/var/lang/lib/python3.7/imp.py", line 171, in load_source
    module = _load(spec)
  File "<frozen importlib._bootstrap>", line 696, in _load
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/var/task/app.py", line 4, in <module>
    from typing import Optional
  File "/var/task/typing.py", line 1347, in <module>
    class Callable(extra=collections_abc.Callable, metaclass=CallableMeta):
  File "/var/task/typing.py", line 1003, in __new__
    self._abc_registry = extra._abc_registry

Steps to Reproduce the Problem

1. Installed the latest available version of the SDK through pip so it will install all required dependencies too.
2. Create a test.py file and import the Cybersource SDK in an environment with Python > 3.5.
3. Execute the script python or upload the code to Lambda and run it.

Suggestion for Fix

Opened a PR here: https://github.com/CyberSource/cybersource-rest-client-python/pull/21

Reproduce the failure by installing the latest SDK with pip and importing the client or typing.Optional from test.py, using Python 3.7 or Lambda as described. Review the dependency behavior and PR #21; done means the SDK imports successfully on Python 3.7 without the Callable __abc_registry error.