Install the requests-toolbelt library:
https://github.com/sigmavirus24/requests-toolbelt
For App Engine it could be something like: pip install requests-toolbelt -t lib
(See: https://cloud.google.com/appengine/docs/python/tools/using-libraries-python-27#installing_a_library)
Then add:
from requests_toolbelt.adapters import appengine
appengine.monkeypatch()
In your main.py or equivalent.