Remove unneeded import guard (#144)

Author: Jon Wayne Parrott

additional_packages/google_auth_oauthlib/google_auth_oauthlib/helpers.py

@@ -23,14 +23,8 @@
 
 import json
 
-try:
-    import requests_oauthlib
-except ImportError:  # pragma: NO COVER
-    raise ImportError(
-        'The requests-oauthlib library is not installed, please install the '
-        'requests-oauthlib package to use google.oauth2.oauthlib.')
-
 import google.oauth2.credentials
+import requests_oauthlib
 
 _REQUIRED_CONFIG_KEYS = frozenset(('auth_uri', 'token_uri', 'client_id'))