refactor: drop support for Python 2, drop requirement for six package

[whtsky]

No description provided.

[codecov]

Codecov Report

Merging #539 into master will decrease coverage by 1.05%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
- Coverage   90.32%   89.26%   -1.06%     
==========================================
  Files          28       28              
  Lines        1437     1435       -2     
==========================================
- Hits         1298     1281      -17     
- Misses        139      154      +15

Impacted Files	Coverage Δ
werobot/crypto/exceptions.py	100% <ø> (ø)	⬆️
werobot/crypto/__init__.py	92.85% <ø> (-0.13%)	⬇️
werobot/parser.py	100% <ø> (ø)	⬆️
werobot/logger.py	50% <0%> (+5.1%)	⬆️
werobot/messages/messages.py	100% <100%> (ø)	⬆️
werobot/utils.py	100% <100%> (ø)	⬆️
werobot/messages/events.py	100% <100%> (ø)	⬆️
werobot/robot.py	86.75% <100%> (-0.57%)	⬇️
werobot/client.py	98.07% <100%> (+0.03%)	⬆️
werobot/session/saekvstorage.py	46.66% <0%> (-53.34%)	⬇️
... and 11 more

---

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[TravisBuddy]

Hey @whtsky,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: 6b797070-558a-11ea-850c-e5458713aaa0

[TravisBuddy]

Hey @whtsky,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: b6099c80-5596-11ea-850c-e5458713aaa0

[whtsky]

好像不能把非 Draft PR 转成 Draft

[TravisBuddy]

Hey @whtsky,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: 3070e930-562b-11ea-8ad8-19cca21d7143

[helloqiu]

感觉是不是还得更新一下文档和 CHANGELOG

[TravisBuddy]

Hey @whtsky,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: 908d6020-59ad-11ea-822e-0bf67ba4204b

[whtsky]

周末看