Notes about GAE Google App Engine - python-telegram-bot/python-telegram-bot GitHub Wiki
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Make sure to add this block to app.yaml
(see 1393):
env_variables:
GAE_USE_SOCKETS_HTTPLIB : 'true'
Note: This environmental variable applies only to applications developed for the Standard Environment and using Python 2.7 (see https://cloud.google.com/appengine/docs/standard/python/sockets/)