Simplekml

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**Fixes**
* Fixed adding unnecessary tag (gxlabelvisibility) when it is not being used.

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**Fixes**
* Fixed missing gxlabelvisibility property from :class:`simplekml.LineStyle`.

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**Fixes**
* Fixed syntax error for Python 3

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**Fixes**
* Fixed missing gxvieweroptions property from :class:`simplekml.Camera` and :class:`simplekml.LookAt`.

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**Fixes**
* Fixed unicode errors where some KML elements were causing an error when using unicode.

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**Fixes**
* Changed added model to :class:`simplekml.GxTrack`
* Added __version__ property