Upgrade python-slugify to 3.0.2

[BKPepe]

Description:

Devs from python-slugify these days released a few versions recently. The latest version is 3.0.2 from 31st March 2019. HA has a requirement for version 1.2.6 from 2nd September 2018.

Changelog: https://github.com/un33k/python-slugify/blob/master/CHANGELOG.md

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

[homeassistant]

Hi @BKPepe,

It seems you haven't yet signed a CLA. Please do so here.

Once you do that we will be able to review and accept this pull request.

Thanks!