Tag v0.2.7

[tomfitzhenry]

pypi lists a gatt version 0.2.7: https://pypi.org/project/gatt/0.2.7/

I see this repo doesn't have a 0.2.7 tag. If you could add the right tag, that'd be handy and allow NixOS to upgrade from 0.2.6 to 0.2.7.