PR for v.1.10.0

[hcpchris]

• ENHANCEMENT: Added User-Agent and Referer support to the ExportServer class to accommodate
  new security measures on Highsoft's public Export Server instance.
• BUGFIX: Adjusted handling of NumPy datetime64 values to serialize them to Unix epoch-based milliseconds,
  rather than the default nanoseconds (closes Creating a timeseries chart from pandas parses Timestamp into nanoseconds rather than milliseconds #204).
• DOCS: Added examples of adjusting datetime64 values to epoch-based milliseconds (courtesy of @ThomasGl).

[codecov]

Codecov Report

Attention: Patch coverage is 76.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 87.92%. Comparing base (7b12281) to head (ead44f7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
highcharts_core/headless_export.py	80.00%	2 Missing and 3 partials ⚠️
highcharts_core/utility_functions.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   87.95%   87.92%   -0.04%     
==========================================
  Files         210      210              
  Lines       31166    31195      +29     
  Branches     2553     2402     -151     
==========================================
+ Hits        27411    27427      +16     
- Misses       2793     2800       +7     
- Partials      962      968       +6     

Flag	Coverage Δ
	?

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.