Improve etcd comapabitity of multiple versions

[Revolution1]

#41

[codecov]

Codecov Report

Merging #42 into master will decrease coverage by 0.15%.
The diff coverage is 87.23%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   91.14%   90.99%   -0.16%     
==========================================
  Files          45       45              
  Lines        2609     2631      +22     
  Branches      270      273       +3     
==========================================
+ Hits         2378     2394      +16     
- Misses        142      145       +3     
- Partials       89       92       +3

Impacted Files	Coverage Δ
etcd3/client.py	88.18% <0%> (-1.73%)	⬇️
tests/test_py3/test_aio_client.py	100% <100%> (ø)	⬆️
etcd3/stateful/watch.py	83.87% <100%> (+0.22%)	⬆️
tests/envs.py	100% <100%> (ø)	⬆️
tests/test_py3/test_aio_auth_apis.py	93.33% <80%> (-6.67%)	⬇️
tests/test_auth_apis.py	97.05% <89.47%> (-2.95%)	⬇️

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