Setuptools

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Changes
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* 2875: Introduce changes from pypa/distutils514e9d0, including support for overrides from Debian and pkgsrc, unlocking the possibility of making SETUPTOOLS_USE_DISTUTILS=local the default again.

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Misc
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* 2885: Fixed errors when encountering LegacyVersions.

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Changes
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* 2497: Update packaging to 21.2.
* 2877: Back out deprecation of setup_requires and replace instead by a deprecation of setuptools.installer and fetch_build_egg. Now setup_requires is still supported when installed as part of a PEP 517 build, but is deprecated when an unsatisfied requirement is encountered.
* 2879: Bump packaging to 21.2.

Documentation changes
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* 2867: PNG/ICO images replaced with SVG in the docs.
* 2867: Added support to SVG "favicons" via "in-tree" Sphinx extension.

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Misc
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* 2880: Removed URL requirement for ``pytest-virtualenv`` in ``setup.cfg``.
PyPI rejects packages with dependencies external to itself.
Instead the test dependency was overwritten via ``tox.ini``

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Deprecations
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* 2856: Support for custom commands that inherit directly from ``distutils`` is
**deprecated**. Users should extend classes provided by setuptools instead.

Breaking Changes
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* 2870: Started failing on invalid inline description with line breaks :class:`ValueError` -- by :user:`webknjaz`

Changes
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* 2698: Exposed exception classes from ``distutils.errors`` via ``setuptools.errors``.
* 2866: Incorporate changes from pypa/distutilsf1b0a2b.

Documentation changes
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* 2227: Added sphinx theme customisations to display the new logo in the sidebar and
use its colours as "accent" in the documentation -- by :user:`abravalheri`
* 2227: Added new setuptools logo, including editable files and artwork documentation
-- by :user:`abravalheri`
* 2698: Added mentions to ``setuptools.errors`` as a way of handling custom command
errors.
* 2698: Added instructions to migrate from ``distutils.commands`` and
``distutils.errors`` in the porting guide.
* 2871: Added a note to the docs that it is possible to install
``setup.py``-less projects in editable mode with :doc:`pip v21.1+
<pip:index>`, only having ``setup.cfg`` and ``pyproject.toml`` in
project root -- by :user:`webknjaz`

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Misc
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* 2849: Add fallback for custom ``build_py`` commands inheriting directly from
:mod:`distutils`, while still handling ``include_package_data=True`` for
``sdist``.