Update packages and drop python2 support

[tmccombs]

[tmccombs]

The build failed on 3.5 because one of the updated dependencies (zipp) isn't compatible with python 3.5. Should python 3.5 continue to be part if the build? If so, is there a way to run pip-multi-compile to update the requirements files with constraints on the python version? Or maybe even have different requirements files based on the version?

I don't know why the installdeps task is timing out on python 3.6 and python 3.7.

[codecov-io]

Codecov Report

Merging #1126 into master will decrease coverage by 0.20%. The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #1126      +/-   ##
==========================================
- Coverage   94.02%   93.82%   -0.21%     
==========================================
  Files          10       10              
  Lines         787      826      +39     
==========================================
+ Hits          740      775      +35     
- Misses         47       51       +4     

Impacted Files	Coverage Δ
etcd3/exceptions.py	88.88% <50.00%> (-11.12%)	:arrow_down:
etcd3/locks.py	95.45% <94.28%> (-2.63%)	:arrow_down:
etcd3/__init__.py	100.00% <100.00%> (ø)
etcd3/client.py	96.52% <100.00%> (+0.15%)	:arrow_up:
etcd3/utils.py	100.00% <100.00%> (ø)
etcd3/watch.py	85.61% <0.00%> (-0.21%)	:arrow_down:
etcd3/leases.py	100.00% <0.00%> (ø)
etcd3/members.py	81.25% <0.00%> (+1.25%)	:arrow_up:

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