Update libsass to 0.20.0

[pyup-bot]

This PR updates libsass from 0.19.4 to 0.20.0.

Changelog

0.20.0

--------------

Released on November 3, 2019.

- Produce abi3 wheels on macos / linux [:issue:`307` by Anthony Sottile]
- Follow up the libsass upstream: 3.6.4 --- See the release notes of LibSass
3.6.4__. [:issue:`313` by Anthony Sottile]

Links

• PyPI: https://pypi.org/project/libsass
• Changelog: https://pyup.io/changelogs/libsass/
• Repo: https://github.com/sass/libsass-python/releases