* Update default scope to be iterable for both py2 and py3

Encounter this issue( google.auth.exceptions.RefreshError: ('invalid_scope: h is not a valid audience string.', u'{\n  "error": "invalid_scope",\n  "error_description": "h is not a valid audience string."\n}')) which stackoverflow shows https://stackoverflow.com/questions/48332352/google-server-to-server-oauth-error.

* Update bigquery_metadata_extractor.py

* Update bigquery_metadata_extractor.py

* Update bigquery_metadata_extractor.py

* Update bigquery_metadata_extractor.py