improve testing, logging, transport and exceptions

[yufeiminds]

ENHANCEMENTS:

• Improve testing with requests_mock
• Add InvalidResponseException and HTTPStatusException
• Add request_uuid to response and exception logging
• Add default exception middileware
• Change documentation url

BUG FIXES:

• Fix pytest version compatible

[codecov-commenter]

Codecov Report

Merging #56 into master will increase coverage by 3.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   93.88%   96.89%   +3.01%     
==========================================
  Files          19       19              
  Lines         572      612      +40     
==========================================
+ Hits          537      593      +56     
+ Misses         35       19      -16     

Impacted Files	Coverage Δ
ucloud/core/exc/__init__.py	100.00% <ø> (ø)
ucloud/core/client/_client.py	100.00% <100.00%> (+3.79%)	⬆️
ucloud/core/exc/_exc.py	100.00% <100.00%> (ø)
ucloud/core/transport/_requests.py	93.75% <100.00%> (+11.78%)	⬆️
ucloud/core/transport/http.py	95.45% <100.00%> (+5.79%)	⬆️
ucloud/core/auth/_cfg.py	100.00% <0.00%> (+3.44%)	⬆️
ucloud/core/utils/middleware.py	100.00% <0.00%> (+14.28%)	⬆️

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