Migrate CI to GitHub Actions

[messense]

Closes #525

TODO:

• Windows CI
• Codecov
• Auto release to PyPI

[codecov-io]

Codecov Report

Merging #528 into master will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #528      +/-   ##
==========================================
+ Coverage   71.77%   72.07%   +0.29%     
==========================================
  Files          81       81              
  Lines        3104     3137      +33     
==========================================
+ Hits         2228     2261      +33     
  Misses        876      876

Flag	Coverage Δ
#unittests	72.07% <ø> (?)

Impacted Files	Coverage Δ
wechatpy/client/api/base.py	100% <0%> (ø)	⬆️
wechatpy/enterprise/client/__init__.py	100% <0%> (ø)	⬆️
wechatpy/client/api/datacube.py	98.43% <0%> (+0.02%)	⬆️
wechatpy/component.py	70.43% <0%> (+0.19%)	⬆️
wechatpy/fields.py	96.62% <0%> (+0.2%)	⬆️
wechatpy/client/base.py	82.08% <0%> (+0.27%)	⬆️
wechatpy/enterprise/events.py	94.52% <0%> (+0.31%)	⬆️
wechatpy/pay/__init__.py	65.75% <0%> (+0.47%)	⬆️
wechatpy/client/api/qrcode.py	83.33% <0%> (+0.98%)	⬆️
wechatpy/crypto/pkcs7.py	88.88% <0%> (+1.38%)	⬆️
... and 1 more

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[messense]

Windows CI 先不弄了。