1.6.0

Last Python2 support, compatibility for recent Click versions and improve
performance

This will be the last version with Python2 support. A next release will come
soon to remove its support and focus on Python3 only. This will may also drop
support for old Click and libsass versions.

Click requirement has been relaxed to only require for version greater or equal
to 5.1.0. This has been currently tested to 7.1.2 so it's surely safe
from 5.x to 7.x versions.

Watcher has been modified to avoid performing indexation on every event and
every file. It should give a few performance improvements and also avoid a bug
with editors making a transition writing file when saving modification (like
a foo.scss.part when writing on foo.scss) which may leaded to incorrect
errors.