PR for v.1.9.0

[hcpchris]

• BUGFIX: Fixed missing serialization/de-serialization of ChartEvents.render.
• BUGFIX: Added new utility_classes.data_labels.PieDataLabel class to ensure support for
  the .distance property on Pie-chart (and descended) data labels. Closes distance keyword for data_labels has no effect in pie charts #183.
• BUGFIX: Fixed options.chart.height type validation to accept string values as per JS API.
  (Issue reported in Missing keywords for Sunburst chart #184).
• BUGFIX: Added missing support for options.plot_options.sunburst.SunburstOptions.border_radius
  (issue reported in Missing keywords for Sunburst chart #184).
• BUGFIX: Added support for concatenation via + operator in JS literal strings when parsed by
  .from_js_literal(). Closes JS literal containing + for string concatenation breaks from_js_literal() #185.
• ENHANCEMENT: Added support for utility_classes.border_radius.BorderRadius object.

[codecov]

Codecov Report

Attention: Patch coverage is 58.02469% with 68 lines in your changes missing coverage. Please review.

Project coverage is 88.02%. Comparing base (ed56ef9) to head (2897f01).
Report is 5 commits behind head on develop.

Files	Patch %	Lines
highcharts_core/utility_classes/border_radius.py	41.07%	33 Missing ⚠️
highcharts_core/options/plot_options/pie.py	56.52%	9 Missing and 1 partial ⚠️
highcharts_core/options/plot_options/sunburst.py	47.36%	9 Missing and 1 partial ⚠️
highcharts_core/utility_classes/data_labels.py	75.00%	6 Missing and 1 partial ⚠️
highcharts_core/js_literal_functions.py	69.23%	2 Missing and 2 partials ⚠️
highcharts_core/options/plot_options/bar.py	73.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #188      +/-   ##
===========================================
- Coverage    88.16%   88.02%   -0.15%     
===========================================
  Files          209      210       +1     
  Lines        30990    31144     +154     
  Branches      2523     2543      +20     
===========================================
+ Hits         27322    27413      +91     
- Misses        2717     2773      +56     
- Partials       951      958       +7     

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