py-vdirsyncer: update to 0.16.6.

Version 0.16.6
==============

*released on 13 June 2018*

- **Packagers:** Documentation building no longer needs a working installation
  of vdirsyncer.

Version 0.16.5
==============

*released on 13 June 2018*

- **Packagers:** click-log 0.3 is required.
- All output will now happen on stderr (because of the upgrade of ``click-log``).

(wiz)