Iroin-python-pptx

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- Add Chart.font
- Fix 293 Can't hide title of single-series Chart
- Fix shape.width value is not type Emu
- Fix add a:defRPr with c:rich (fixes some font inheritance breakage)

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- Add Picture.auto_shape_type
- Remove Python 2.6 testing from build
- Update dependencies to avoid vulnerable Pillow version
- Fix 260, 301, 382, 401
- Add _Paragraph.add_line_break()
- Add Connector.line

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- Add gradient fill.
- Add experimental "turbo-add" option for producing large shape-count slides.

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- Add `shape.shadow` property to autoshape, connector, picture, and group
shape, returning a `ShadowFormat` object.
- Add `ShadowFormat` object with read/write (boolean) `.inherit` property.
- Fix 328 add support for 26+ series in a chart

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- Add `Picture.crop_x` setters, allowing picture cropping values to be set,
in addition to interrogated.
- Add `Slide.background` and `SlideMaster.background`, allowing the
background fill to be set for an individual slide or for all slides based
on a slide master.
- Add option `shapes` parameter to `Shapes.add_group_shape`, allowing a group
shape to be formed from a number of existing shapes.
- Improve efficiency of `Shapes._next_shape_id` property to improve
performance on high shape-count slides.

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- Add `GroupShape`, providing properties specific to a group shape, including
its `shapes` property.
- Add `GroupShapes`, providing access to shapes contained in a group shape.
- Add `SlideShapes.add_group_shape()`, allowing a group shape to be added to
a slide.
- Add `GroupShapes.add_group_shape()`, allowing a group shape to be added to
a group shape, enabling recursive, multi-level groups.
- Add support for adding jump-to-named-slide behavior to shape and run
hyperlinks.