Release Version 0.6.0 Broken Due to google-auth version

[ngklingler]

Environment details

• OS type and version: python:3.7-slim-buster Docker image
• Python version: 3.7.10
• pip version: pip 20.2.2 from /usr/local/lib/python3.7/site-packages/pip (python 3.7)
• google-auth-oauthlib version: Version: 0.6.0

Steps to reproduce

1. install version 0.6.0
2. open a python interpreter
3. run import google_auth_oauthlib

Code example

import google_auth_oauthlib

Stack trace

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/airflow/.local/lib/python3.7/site-packages/google_auth_oauthlib/__init__.py", line 21, in <module>
    from .interactive import get_user_credentials
  File "/home/airflow/.local/lib/python3.7/site-packages/google_auth_oauthlib/interactive.py", line 27, in <module>
    import google_auth_oauthlib.flow
  File "/home/airflow/.local/lib/python3.7/site-packages/google_auth_oauthlib/flow.py", line 69, in <module>
    import google_auth_oauthlib.helpers
  File "/home/airflow/.local/lib/python3.7/site-packages/google_auth_oauthlib/helpers.py", line 27, in <module>
    from google.auth import external_account_authorized_user
ImportError: cannot import name 'external_account_authorized_user' from 'google.auth' (/home/airflow/.local/lib/python3.7/site-packages/google/auth/__init__.py)

I believe this is due to this commit which was included in today's release of version 0.6.0. That commit depends on functionality only found in version >=2.13.0 of the google-auth library, and specifies that version in testing constraints, but not in setup.py, where only >= 1.0.0 is required. I will open a PR to fix this momentarily.