Log Message:
py-vdirsyncer: update to 0.16.8.

Version 0.16.8
==============

*released 09 June 2020*

- Support Python 3.7 and 3.8.

This release is functionally identical to 0.16.7.
It's been tested with recent Python versions, and has been marked as supporting
them. It will also be the final release supporting Python 3.5 and 3.6.