Versioned api

[Revolution1]

• support api versioning
• put json specs into .py files to improve load speed and pyinstaller compability

[codecov]

Codecov Report

Merging #51 into master will increase coverage by 0.03%.
The diff coverage is 92.39%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   91.13%   91.17%   +0.03%     
==========================================
  Files          47       51       +4     
  Lines        2674     2786     +112     
  Branches      273      286      +13     
==========================================
+ Hits         2437     2540     +103     
- Misses        145      147       +2     
- Partials       92       99       +7

Impacted Files	Coverage Δ
etcd3/apis/__init__.py	100% <ø> (ø)	⬆️
tests/test_py3/test_aio_auth_apis.py	100% <ø> (+6.66%)	⬆️
etcd3/apis/cluster.py	33.33% <0%> (ø)	⬆️
etcd3/apis/watch.py	84% <100%> (ø)	⬆️
etcd3/apis/lock.py	100% <100%> (ø)	⬆️
etcd3/apis/maintenance.py	100% <100%> (ø)	⬆️
tests/test_swagger_helper.py	100% <100%> (ø)	⬆️
etcd3/stateful/lock.py	82.63% <100%> (-0.1%)	⬇️
etcd3/apis/auth.py	92.68% <100%> (ø)	⬆️
etcd3/errors/errors.py	100% <100%> (ø)	⬆️
... and 25 more

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