Setuptools

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Changes
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* 479: Remove some usage of the deprecated ``imp`` module.

Misc
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* 1565: Changed html_sidebars from string to list of string as per
https://www.sphinx-doc.org/en/master/changes.html#id58

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Misc
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* 1697: Moved most of the constants from setup.py to setup.cfg
* 1749: Fixed issue with the PEP 517 backend where building a source distribution would fail if any tarball existed in the destination directory.
* 1750: Fixed an issue with PEP 517 backend where wheel builds would fail if the destination directory did not already exist.
* 1756: Force metadata-version >= 1.2. when project urls are present.
* 1769: Improve ``package_data`` check: ensure the dictionary values are lists/tuples of strings.
* 1788: Changed compatibility fallback logic for ``html.unescape`` to avoid accessing ``HTMLParser.unescape`` when not necessary. ``HTMLParser.unescape`` is deprecated and will be removed in Python 3.9.
* 1790: Added the file path to the error message when a ``UnicodeDecodeError`` occurs while reading a metadata file.

Documentation changes
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* 1776: Use license classifiers rather than the license field.

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Changes
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* 1671: Fixed issue with the PEP 517 backend that prevented building a wheel when the ``dist/`` directory contained existing ``.whl`` files.
* 1709: In test.paths_on_python_path, avoid adding unnecessary duplicates to the PYTHONPATH.
* 1741: In package_index, now honor "current directory" during a checkout of git and hg repositories under Windows

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Breaking Changes
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* 1735: When parsing setup.cfg files, setuptools now requires the files to be encoded as UTF-8. Any other encoding will lead to a UnicodeDecodeError. This change removes support for specifying an encoding using a 'coding: ' directive in the header of the file, a feature that was introduces in 40.7. Given the recent release of the aforementioned feature, it is assumed that few if any projects are utilizing the feature to specify an encoding other than UTF-8.

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Changes
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* 1675: Added support for ``setup.cfg``-only projects when using the ``setuptools.build_meta`` backend. Projects that have enabled PEP 517 no longer need to have a ``setup.py`` and can use the purely declarative ``setup.cfg`` configuration file instead.
* 1720: Added support for ``pkg_resources.parse_requirements``-style requirements in ``setup_requires`` when ``setup.py`` is invoked from the ``setuptools.build_meta`` build backend.
* 1664: Added the path to the ``PKG-INFO`` or ``METADATA`` file in the exception
text when the ``Version:`` header can't be found.

Documentation changes
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* 1705: Removed some placeholder documentation sections referring to deprecated features.

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Changes
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* 1652: Added the ``build_meta:__legacy__`` backend, a "compatibility mode" PEP 517 backend that can be used as the default when ``build-backend`` is left unspecified in ``pyproject.toml``.
* 1635: Resource paths are passed to ``pkg_resources.resource_string`` and similar no longer accept paths that traverse parents, that begin with a leading ``/``. Violations of this expectation raise DeprecationWarnings and will become errors. Additionally, any paths that are absolute on Windows are strictly disallowed and will raise ValueErrors.
* 1536: ``setuptools`` will now automatically include licenses if ``setup.cfg`` contains a ``license_file`` attribute, unless this file is manually excluded inside ``MANIFEST.in``.