Pip-compile-multi

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* Update dependencies.
* Revert relative path normalization, introduced in 167 (thanks to john-bodley 200).

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* Drop Python 2.7 support. pip-tools 4 no longer works with the latest pip,
there's no way to continue Python 2.7 support.

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* Remove directory path from "via" annotations (thanks to HALtheWise 166 167).

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* Add option ``--annotate-index`` (thanks to john-bodley 160).

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* Enable accidentially disabled ``--upgrade`` option.

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* Minor fixes to packaging and documentation.

Warning: this version is broken and won't pass ``--upgrade`` option to ``pip-compile``.
If you have this version installed, you need to manually upgrade it.
For example, using command::

pip-compile-multi --upgrade-package pip-compile-multi

Like in this `PR <https://github.com/mozilla-releng/shipit/pull/1>`_.