Python-pptx

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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.

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- Add `SlideShapes.build_freeform()`, allowing freeform shapes (such as maps)
to be specified and added to a slide.
- Add support for patterned fills.
- Add `LineFormat.dash_style` to allow interrogation and setting of dashed
line styles.

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- Add `SlideShapes.add_movie()`, allowing video media to be added to a slide.

- fix 190 Accommodate non-conforming part names having '00' index segment.
- fix 273 Accommodate non-conforming part names having no index segment.
- fix 277 ASCII/Unicode error on non-ASCII multi-level category names
- fix 279 BaseShape.id warning appearing on placeholder access.