Pin urlib3 version

[debraj-manna]

Related to psf/requests#4830 .

Issue - #829

[CyberDem0n]

Unfortunately this is the same problem as it was last summer: #693 and my opinion on it didn't change.
Urllib3 is a dependency for thousands of projects and modules. Pinning the version of urllib3 will just increase chances of dependency hell and failing travis confirms it. If you first it install patroni and on the next step install something else with the different version of urllib3, it will happily upgrade/downgrade it.

Requirement already up-to-date: urllib3==1.23 in /home/travis/virtualenv/python2.7.14/lib/python2.7/site-packages (from -r requirements.txt (line 1))
...
Collecting urllib3[secure]; python_version < "3" (from coveralls)
  Downloading https://files.pythonhosted.org/packages/8c/4b/5cbc4cb46095f369117dcb751821e1bef9dd86a07c968d8757e9204c324c/urllib3-1.24-py2.py3-none-any.whl (117kB)
    100% |████████████████████████████████| 122kB 3.8MB/s 

As a result all acceptance tests are failing.
Switching the order of installing patroni requirements and test requirements will help to pass tests, but it won't really help to improve the situation in general.