PR for v.1.10.2

[hcpchris]

• BUGFIX: Fixed type coercion for plot_options.series.data_labels to allow for series type-specific
  label label classes. Closes BUG: Silently ignoring distance property for data Labels in pie charts #209.
• BUGFIX: Modified the default Referer and User-Agent strings for use when exporting charts.
• DOCS: Added some additional documentation around the User-Agent and Referer support for use when
  exporting charts.
• ENHANCEMENT: Some minor refactoring.

[codecov]

Codecov Report

Attention: Patch coverage is 66.16052% with 156 lines in your changes missing coverage. Please review.

Project coverage is 87.89%. Comparing base (95874cf) to head (8a036c5).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
highcharts_core/chart.py	55.07%	60 Missing and 2 partials ⚠️
highcharts_core/headless_export.py	57.85%	50 Missing and 9 partials ⚠️
highcharts_core/options/plot_options/series.py	76.92%	15 Missing and 6 partials ⚠️
highcharts_core/utility_classes/data_labels.py	84.78%	9 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
- Coverage   87.92%   87.89%   -0.03%     
==========================================
  Files         210      210              
  Lines       31195    31225      +30     
  Branches     2402     2409       +7     
==========================================
+ Hits        27427    27445      +18     
- Misses       2800     2808       +8     
- Partials      968      972       +4     

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