Pytoshop

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Bugfixes:

- Fix 4: Change masked layer to same size as image layer when using
the `user.nested_layers` interface.

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- Declared API as stable.

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Minor improvements:

- Make it easier to choose the right channel, given the current color
mode.

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Minor improvements:

- The color assigned to a layer (displayed in the layer list) is now
available in the ``nested_layers`` API.

Bugfixes:

- Fix crash in ``GuideResourceBlock``.

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Minor improvements:

- Parse multiple values in a struct together when possible. This has
a modest speed improvement.

Bugfixes:

- Fix pip install [1]

- The main canvas image will use the specified compression algorithm
when using nested_layers_to_psd.

- Non-image layers correctly set "pixel_data_is_irrelevant" flag.

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Improvements:

- For speed purposes, pytoshop no longer uses traitlets.

- Performance improvements to the compression/decompression code.

- Added support for the ``shmd`` metadata tagged block, and the ability
to access it from the ``user.nested_layers`` API.

Bugfixes:

- Updated the list of tagged blocks that use 8-bit lengths.

- Fixed a bug where the image data would be corrupted when writing
images from an input file to an output file with a different file
format version.

- Fixed a crash when the input file contains no layer group ids.

- Allow Numpy arrays of shape () in place of scalars for constant
images.