Setuptools

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Changes
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* 2608: Added informative error message to PEP 517 build failures owing to
an empty ``setup.py`` -- by :user:`layday`

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No significant changes.

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Misc
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* 2595: Reduced scope of dash deprecation warning to Setuptools/distutils only -- by :user:`melissa-kun-li`

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Documentation changes
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* 2584: Added ``sphinx-inline-tabs`` extension to allow for comparison of ``setup.py`` and its equivalent ``setup.cfg`` -- by :user:`amy-lei`

Misc
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* 2592: Made option keys in the ``[metadata]`` section of ``setup.cfg`` case-sensitive. Users having
uppercase option spellings will get a warning suggesting to make them to lowercase
-- by :user:`melissa-kun-li`

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Changes
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* 1608: Removed the conversion of dashes to underscores in the :code:`extras_require` and :code:`data_files` of :code:`setup.cfg` to support the usage of dashes. Method will warn users when they use a dash-separated key which in the future will only allow an underscore. Note: the method performs the dash to underscore conversion to preserve compatibility, but future versions will no longer support it -- by :user:`melissa-kun-li`

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Breaking Changes
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* 2582: Simplified build-from-source story by providing bootstrapping metadata in a separate egg-info directory. Build requirements no longer include setuptools itself. Sdist once again includes the pyproject.toml. Project can no longer be installed from source on pip 19.x, but install from source is still supported on pip < 19 and pip >= 20 and install from wheel is still supported with pip >= 9.

Changes
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* 1932: Handled :code:`AttributeError` by raising :code:`DistutilsSetupError` in :code:`dist.check_specifier()` when specifier is not a string -- by :user:`melissa-kun-li`
* 2570: Correctly parse cmdclass in setup.cfg.

Documentation changes
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* 2553: Added userguide example for markers in extras_require -- by :user:`pwoolvett`