Ubuntu下安装kivy
参考：
https://stackoverflow.com/questions/45476686/kivy-critical-app-unable-to-get-a-window-abort
step 1:
sudo add-apt-repository ppa:kivy-team/kivy
step 2:
sudo apt-get update
step 3:
python2:sudo apt-get install python-kivy
python3:sudo apt-get install python3-kivy
*step 4:
sudo apt-get install kivy-examples
安装后出现如下问题时，解决方法：sudo apt-get install libsdl2-2.0-0 libsdl2-image-2.0-0 libsdl2-mixer-2.0-0 libsdl2-ttf-2.0-0
[CRITICAL] [App ] Unable to get a Window, abort.
