Update alabaster to 0.7.15

[pyup-bot]

This PR updates alabaster from 0.7.8 to 0.7.15.

Changelog

0.7.9

- :feature:`6` (and :issue:`70`, both via :issue:`84`) Make all remaining
hardcoded style colors configurable, plus related cleanup (such as improving
differentiation of some admonition blocks such as ``warn`` and ``note``,
ensuring generic admonitions are left untouched, etc). Credit:
``ShadowKyogre``.
- :feature:`83` Expose Sphinx's toctree ``collapse`` option as the new
``sidebar_collapse`` config option. Credit: Eric Holscher.
- :feature:`80` Add support for ``<link rel="canonical">`` (i.e. canonical
URLs). Thanks to Ben Gamari for the patch.
- :support:`7` Generate real documentation site, both because the README is
just too big now, and so we can `eat our own dog food
<https://en.wikipedia.org/wiki/Eating_your_own_dog_food>`_.

Links

• PyPI: https://pypi.org/project/alabaster
• Changelog: https://data.safetycli.com/changelogs/alabaster/

[pyup-bot]

Closing this in favor of #1070