0.9.0 [2020-03-02]

• updated default version of openwisp-controller to 0.5.x
• updated default version of openwisp-network-topology to 0.3.x
• dropped support for python 2 and python 3.5 (minimum python version
  required is 3.6)
• dropped support for Debian 9 and Ubuntu 16 - users using these distros
  are reccomended to upgrade to a newer OS; alternatively they can compile
  and install python 3.6 on their own, the role should continue working
  in the short term
• added support for customizable admin theme