Implementing Python 3 Support

[hiranya911]

• Using the six library to implement Python 2 and 3 cross-version compatibility
• Updating initialize_app() function to use the ApplicationDefault credentials when necessary
• New test cases
• Python3 config for tox

[jwngr]

Just a couple minor comments. Also, the CI failed with this error:

============================= test session starts ==============================
platform linux2 -- Python 2.7.5, pytest-3.0.7, py-1.4.33, pluggy-0.4.0
rootdir: /home/jenkins/.jenkins/jobs/continuous-integration-admin-sdk-python-public/workspace, inifile:
collected 135 items

tests/test_app.py .....F........................................
tests/test_auth.py .................................................................................
tests/test_credentials.py ........

=================================== FAILURES ===================================
___________ TestFirebaseApp.test_non_default_app_init[refreshtoken] ____________

self = <tests.test_app.TestFirebaseApp object at 0x2ccf890>
app_credential = None

    def test_non_default_app_init(self, app_credential):
>       app = firebase_admin.initialize_app(app_credential, name='myApp')

tests/test_app.py:87: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
firebase_admin/__init__.py:38: in initialize_app
    credential = credentials.ApplicationDefault()
firebase_admin/credentials.py:96: in __init__
    self._g_credential = client.GoogleCredentials.get_application_default()
../../../../.local/lib/python2.7/site-packages/oauth2client/client.py:1264: in get_application_default
    return GoogleCredentials._get_implicit_credentials()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cls = <class 'oauth2client.client.GoogleCredentials'>

    @classmethod
    def _get_implicit_credentials(cls):
        """Gets credentials implicitly from the environment.
    
            Checks environment in order of precedence:
            - Environment variable GOOGLE_APPLICATION_CREDENTIALS pointing to
              a file with stored credentials information.
            - Stored "well known" file associated with `gcloud` command line tool.
            - Google App Engine (production and testing)
            - Google Compute Engine production environment.
    
            Raises:
                ApplicationDefaultCredentialsError: raised when the credentials
                                                    fail to be retrieved.
            """
        # Environ checks (in order).
        environ_checkers = [
            cls._implicit_credentials_from_files,
            cls._implicit_credentials_from_gae,
            cls._implicit_credentials_from_gce,
        ]
    
        for checker in environ_checkers:
            credentials = checker()
            if credentials is not None:
                return credentials
    
        # If no credentials, fail.
>       raise ApplicationDefaultCredentialsError(ADC_HELP_MSG)
E       ApplicationDefaultCredentialsError: The Application Default Credentials are not available. They are available if running in Google Compute Engine. Otherwise, the environment variable GOOGLE_APPLICATION_CREDENTIALS must be defined pointing to a file defining the credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.

../../../../.local/lib/python2.7/site-packages/oauth2client/client.py:1254: ApplicationDefaultCredentialsError
===================== 1 failed, 134 passed in 1.45 seconds =====================

I assume we also need to update the CI to run the tests in both 2.7 and 3.3 now, right?

[jwngr]

Nit: add two spaces more before the # sign to easily distinguish the comment from the command.

[hiranya911]

Done

[jwngr]

Suggestion: drop "the Python Admin SDK on."
Side note: the official terminology we use is "Admin SDK"

[hiranya911]

Done

[jwngr]

Add a new line.

[hiranya911]

Done

[hiranya911]

Made the suggested changed. The CI error was due to a bug in the code (missing return statement from a test fixture), which I have fixed. We should update our CI scripts to use tox soon. But before that I'd like to add a couple more environments to the tox configuration (at least GAE). We can setup the Jenkins server accordingly afterwards.

[jwngr]

LGTM