Backport from Chainer v7rc

[disktnk]

~Please merge #247 first~ merged, thx!

• backport PRs from Chainer v7rc
  • support ONNX 1.6, opset 11
  • support Permutate
  • and other fixing bugs and typo
• add CI job with ONNX 1.5.0
• simplified README.md
• follow flake8 ignore setting with Chainer

[codecov-io]

Codecov Report

Merging #248 into master will increase coverage by 0.41%. The diff coverage is 95.55%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
+ Coverage   90.62%   91.04%   +0.41%     
==========================================
  Files          23       23              
  Lines        1621     1708      +87     
==========================================
+ Hits         1469     1555      +86     
- Misses        152      153       +1

Impacted Files	Coverage Δ
onnx_chainer/functions/converter.py	100% <ø> (ø)	:arrow_up:
onnx_chainer/mapping.py	90% <ø> (ø)	:arrow_up:
onnx_chainer/functions/math.py	89.67% <100%> (+0.24%)	:arrow_up:
onnx_chainer/functions/__init__.py	100% <100%> (ø)	:arrow_up:
onnx_chainer/functions/activation.py	82.19% <100%> (+1.59%)	:arrow_up:
onnx_chainer/export.py	93.33% <100%> (+0.02%)	:arrow_up:
onnx_chainer/export_testcase.py	94.28% <100%> (+0.34%)	:arrow_up:
onnx_chainer/functions/pooling.py	89.33% <100%> (+0.6%)	:arrow_up:
onnx_chainer/onnx_helper.py	97.72% <80%> (-2.28%)	:arrow_down:
onnx_chainer/replace_func.py	91.83% <93.75%> (+0.27%)	:arrow_up:
... and 2 more

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[disktnk]

/test

[pfn-ci-bot]

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