Toyplot 0.19.0

• First Toyplot release that only supports Python 3.
• Colormaps return the first color instead of the last when the domain is empty.
• The target attribute is supported in rich text hyperlinks, useful for opening hyperlinks in separate tabs from Juypter notebooks.
• Provide an explicit default style for rich text hyperlinks, for consistent default outputs regardless of render context.
• Add rich text support for styling underlines.
• toyplot.locator.Timestamp didn't produce correctly positioned labels in some cases with non-UTC timezones.
• Warn callers who try to use with Python 2.
• Markers and marks are formattable objects, for easier embedding in rich text.
• Switched to an explicit scenegraph to manage relationships among internal objects.
• Fixed a bug that affect the domain and range of shared axes.
• Expanded the user guide section on rendering.
• Include the LICENSE file in source distributions, for downstream maintainers.
• Fixed code that used obsolete Pandas ix API.