[patch] adjust dependencies for python-ectd and patroni

Hi,

I noticed two issues with patroni and python3-etcd packages in CentOS7.

First, the python3-etcd package, installed alone without patroni, is
missing some dependencies:

$ /usr/bin/python3.6 -m pydoc etcd
problem in etcd - ModuleNotFoundError: No module named 'dns'

then installing python36-dns:

$ /usr/bin/python3.6 -m pydoc etcd
problem in etcd - ModuleNotFoundError: No module named 'urllib3'

Then, the base patroni package is missing the urllib3 dependency:

$ patronictl --help
Traceback (most recent call last):
File "/usr/bin/patronictl", line 6, in <module>
from pkg_resources import load_entry_point
File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3095, in <module>
@_call_aside
File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3079, in _call_aside
f(*args, **kwargs)
File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 3108, in _initialize_master_working_set
working_set = WorkingSet._build_master()
File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 570, in _build_master
ws.require(__requires__)
File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 888, in require
needed = self.resolve(parse_requirements(requirements))
File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 774, in resolve
raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'urllib3[secure]!=1.21,>=1.19.1' distribution was not found and is required by patroni
$ /usr/bin/python3.6 -m pydoc patroni.api
problem in patroni.api - ModuleNotFoundError: No module named 'urllib3'

This would be pulled by patroni-etcd, but I believe the dependency
belongs to the base package.

I suggest the attached two patches to fix these issues.

Thanks,
Denis