remove usage of external simplejson

[a-detiste]

simplejson was a backport of Python3.x 'json' to Python2.

It makes little sense to keep it now.

(I learned that today)

[coveralls]

Pull Request Test Coverage Report for Build 10639585465

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.4%) to 88.718%

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Files with Coverage Reduction	New Missed Lines	%
serializable/serializable.py	1	84.78%
serializable/helpers.py	3	88.89%

Totals
Change from base Build 8575045429:	0.4%
Covered Lines:	173
Relevant Lines:	195

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💛 - Coveralls