Property-manager3

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- Added ``set_property()`` and ``clear_property()`` functions.
- Added Python 3.5 to tested and supported versions.
- Rearranged class variables and their documentation (I'm still getting up to
speed with Sphinx, have been doing so for years, probably I'll still be
learning new things a few years from now :-).

.. _Release 1.5: https://github.com/xolox/python-property-manager/compare/1.4...1.5

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- Only inject usage notes when applicable.
- Start using the ``humanfriendly.sphinx`` module.

.. _Release 1.4: https://github.com/xolox/python-property-manager/compare/1.3...1.4

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Support for properties whose values are based on environment variables.

.. _Release 1.3: https://github.com/xolox/python-property-manager/compare/1.2...1.3

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Made it possible to opt out of usage notes.

.. _Release 1.2: https://github.com/xolox/python-property-manager/compare/1.1.1...1.2

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- Made ``repr()`` render only properties of subclasses.
- Removed indentation from doctest formatted code samples in readme.

.. _Release 1.1.1: https://github.com/xolox/python-property-manager/compare/1.1...1.1.1

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- Documented similar projects and distinguishing features.
- Improved the structure of the documentation.

.. _Release 1.1: https://github.com/xolox/python-property-manager/compare/1.0.1...1.1