Steps to reproduce

1. Install latest version on ipv6-only host
2. Try connecting

Expected behaviour

Succesful connection

Actual behaviour

NetworkError is throwed. Mostly of this commit, I think: python-telegram-bot/urllib3@d0a6760

Configuration

Operating System:
Reproducible on debian jessie/ubuntu xenial

Version of Python, python-telegram-bot & dependencies:
python 3.4
All versions >5.3.1 are affected, I believe

Logs

NetworkError: urllib3 HTTPError HTTPSConnectionPool(host='api.telegram.org', port=443): Max retries exceeded with url: /bot326003279:MY_TOKEN_XXX/sendMessage (Caused by NewConnectionError('<telegram.vendor.ptb_urllib3.urllib3.connection.VerifiedHTTPSConnection object at 0x7f6770459750>: Failed to establish a new connection: [Errno -2] Name or service not known',))