Setuptools

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* 1075: Add new ``Description-Content-Type`` metadata field. `See here for
documentation on how to use this field.
<https://packaging.python.org/specifications/#description-content-type>`_

* 1068: Sort files and directories when building eggs for
deterministic order.

* 196: Remove caching of easy_install command in fetch_build_egg.
Fixes issue where ``pytest-runner-N.N`` would satisfy the installation
of ``pytest``.

* 1129: Fix working set dependencies handling when replacing conflicting
distributions (e.g. when using ``setup_requires`` with a conflicting
transitive dependency, fix 1124).

* 1133: Improved handling of README files extensions and added
Markdown to the list of searched READMES.

* 1135: Improve performance of pkg_resources import by not invoking
``access`` or ``stat`` and using ``os.listdir`` instead.

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* 1131: Make possible using several files within ``file:`` directive
in metadata.long_description in ``setup.cfg``.

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* fix 1105: Fix handling of requirements with environment
markers when declared in ``setup.cfg`` (same treatment as
for 1081).

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* 462: Don't assume a directory is an egg by the ``.egg``
extension alone.

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* 1093: Fix test command handler with extras_require.
* 1112, 1091, 1115: Now using Trusty containers in
Travis for CI and CD.

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* 1092: ``pkg_resources`` now uses ``inspect.getmro`` to
resolve classes in method resolution order.