v0.21.0 release
v0.21.0 release

With thanks to all contributors involved in this release:
- anriha
- elParaguayo
- Jaakko Sirén
- Joao Duarte
- Joao Faria Miranda Duarte
- jwijenbergh
- K1llf0rce
- Lars
- matpi
- mcol
- Michael Benford
- MithicSpirit
- modestcap
- nivgsk
- Piotr Machura
- Yutao Yuan

qtile/qtile,ramnes/qtile,ramnes/qtile,qtile/qtile