Use 'simplejson' package if available

[rossmacarthur]

Resolves #60

[codecov-io]

Codecov Report

Merging #63 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #63   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         523    553   +30     
=====================================
+ Hits          523    553   +30

Impacted Files	Coverage Δ
src/serde/model.py	100% <100%> (ø)	⬆️
src/serde/fields.py	100% <0%> (ø)	⬆️

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