Highcharts-core

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* **BUGFIX:** Fixed missing handling of JS module for `HeatmapOptions` and `HeatmapSeries` (closes 207).

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* **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 204).
* **DOCS:** Added examples of adjusting `datetime64` values to epoch-based milliseconds (courtesy of ThomasGL).

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* **BUGFIX:** Fixed ``TypeError`` which occurred on certain ``np.isnan()`` operations when serializing to JS
Literal (courtesy of ThomasGL) (closes 200).

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* **BUGFIX:** Fixed incorrect export serialization of Highcharts Maps for Python map topologies.
* **BUGFIX:** Added support for value ``'allow'`` to ``DataLabel.overflow`` property. Fixes 198.
* **DOCS:** Fixed some typos in documentation (courtesy of JulienBacquart).

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* **BUGFIX:** Resolved over-broad False-y validation of ``PieDataLabel.distance`` to allow for ``0``
values. Closes 192.
* **BUGFIX:** Fixed over-broad False-y validation of ``Tooltip.header_format`` to allow for empty string
values, bringing it in line with ``Tooltip.footer_format``. Closes 193.

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* **ENHANCEMENT:** Added stringified JSON serialization of callback functions when serializing via
``.to_json(for_export = True)``.

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