Fix timeouts when sending large files #533

This issue is in order to track the various tasks of a bug in master branch.
The problem is that urllib3 doesn't honour the read_timeout when using a newly created connection. However, with an existing connection it works just fine.
Regardless of working with upstream urllib3 to fix the issue, we need to fix the issue for v6.0 milestone.

• Create a unitest to recreate the problem. See in travis that the problem is recreated.
• "Vendor" a fork of urllib3 with a fix with python-telegram-bot package.
• Check that the unitest is now fixed.
• Modify defaults of timeouts to sane values:
  • connect_timeout = 5 seconds
  • read_timeout = 5 seconds
  • sendDocument, sendAudio & similar methods = 20 seconds.