Python-petpptx

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- 267 compensate for non-conforming PowerPoint behavior on c:overlay element

- compensate for non-conforming (to spec) PowerPoint behavior related to
c:dLbl/c:tx that results in "can't save" error when explicit data labels
are added to bubbles on a bubble chart.

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- add Chart.chart_title and ChartTitle object
- 263 Use Number type to test for numeric category

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- add DataLabel.font
- add Axis.axis_title

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- add support for NotesSlide (slide notes, aka. notes page)
- add support for arbitrary series ordering in XML
- add Plot.categories providing access to hierarchical categories in an
existing chart.
- add support for date axes on category charts, including writing a dateAx
element for the category axis when ChartData categories are date or
datetime.

**BACKWARD INCOMPATIBILITIES:**

Some changes were made to the boilerplate XML used to create new charts. This
was done to more closely adhere to the settings PowerPoint uses when creating
a chart using the UI. This may result in some appearance changes in charts
after upgrading. In particular:

* Chart.has_legend now defaults to True for Line charts.
* Plot.vary_by_categories now defaults to False for Line charts.

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- add Connector shape type

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- add XY chart types
- add Bubble chart types
- add Radar chart types
- add Area chart types
- add Doughnut chart types
- add Series.points and Point
- add Point.data_label
- add DataLabel.text_frame
- add DataLabel.position
- add Axis.major_gridlines
- add ChartFormat with .fill and .line
- add Axis.format (fill and line formatting)
- add ValueAxis.crosses and .crosses_at
- add Point.format (fill and line formatting)
- add Slide.slide_id
- add Slides.get() (by slide id)
- add Font.language_id
- support blank (None) data points in created charts
- add Series.marker
- add Point.marker
- add Marker.format, .style, and .size