add (python2.7 and python3.5+) Python client for etcd v3, using gRPC-…

[pacoxu]

Here is a Python client for etcd v3.
https://github.com/Revolution1/etcd3-py

Python client for etcd v3 (Using gRPC-JSON-Gateway)

• Free software: Apache Software License 2.0
• Source Code: https://github.com/Revolution1/etcd3-py
• Documentation: https://etcd3-py.readthedocs.io.
• etcd version required: v3.2.2+

Notice: The authentication header through gRPC-JSON-Gateway only supported in etcd v3.3.0+

Features

• Support python2.7 and python3.5+
• Sync client based on requests
• Async client based on aiohttp
• TLS Connection
• support APIs
  • Auth
  • KV
  • Watch
  • Cluster
  • Lease
  • Lock
  • Maintenance
  • Extra APIs
• stateful utilities
  • Watch
  • Lease
  • Transaction
  • Lock

[hexfusion]

LGTM, thanks for the contribution!

[codecov-io]

Codecov Report

Merging #10593 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10593      +/-   ##
==========================================
+ Coverage   71.51%    71.6%   +0.08%     
==========================================
  Files         393      393              
  Lines       36571    36571              
==========================================
+ Hits        26155    26185      +30     
+ Misses       8580     8541      -39     
- Partials     1836     1845       +9

Impacted Files	Coverage Δ
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (-2.78%)	⬇️
clientv3/balancer/balancer.go	84.49% <0%> (-2.33%)	⬇️
etcdserver/api/v2http/client.go	85.51% <0%> (-1.21%)	⬇️
auth/store.go	74.41% <0%> (-0.27%)	⬇️
clientv3/watch.go	91.98% <0%> (-0.22%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (ø)	⬆️
etcdserver/server.go	74.6% <0%> (+0.07%)	⬆️
etcdserver/api/v3rpc/watch.go	83% <0%> (+0.65%)	⬆️
clientv3/leasing/kv.go	89.7% <0%> (+0.66%)	⬆️
... and 9 more

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[hexfusion]

@pacoxu one nit please update git commit per https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#format-of-the-commit-message

Something like Documentation: add Python client for etcd v3 thanks!

[pacoxu]

@hexfusion Change the commit message as you mentioned. Thanks for your comments.