Update default scope to be iterable for both py2 and py3 (#83)

* 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

databuilder/extractor/bigquery_metadata_extractor.py

@@ -34,7 +34,7 @@ class BigQueryMetadataExtractor(Extractor):
     KEY_PATH_KEY = 'key_path'
     PAGE_SIZE_KEY = 'page_size'
     FILTER_KEY = 'filter'
-    _DEFAULT_SCOPES = ('https://www.googleapis.com/auth/bigquery.readonly')
+    _DEFAULT_SCOPES = ['https://www.googleapis.com/auth/bigquery.readonly', ]
     DEFAULT_PAGE_SIZE = 300
     NUM_RETRIES = 3
     DATE_LENGTH = 8