This release is a maintenance-focused one and mostly addresses
compatibility with newer Python versions and newer dependencies.

Note: Version 0.17 has some alpha releases but ultimately was never finalised.
0.18 actually continues where 0.16 left off.

- Support for Python 3.5 and 3.6 has been dropped. This release mostly focuses
  on keeping vdirsyncer compatible with newer environments.
- Python 3.9 is now supported.
- click 8 and click-threading 0.5.0 are now required.
- For those using `pipsi`, we now recommend using `pipx`, it's successor.
- Our Debian/Ubuntu build scripts have been updated. New versions should be
  pushed to those repositories soon.