Grimp

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* Stable release of functionality from 3.0b1-3.

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* Support for independent layers in find_illegal_dependencies_for_layers.

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* Switch to pyproject.toml.
* Rename upstream/downstream in find_illegal_dependencies_for_layers to importer/imported.
The original names were accidentally used in reverse; the new names have less potential for confusion.
* Use Rust extension module for find_illegal_dependencies_for_layers.

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* Log cache activity.
* Drop support for Python 3.7.
* Add find_illegal_dependencies_for_layers method.

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* Change cache filename scheme to use a hash.
* Ignore modules with dots in the filename.

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* Add caching.