Ubuntu下安装kivy

参考：

　　https://kivy.org/doc/stable/installation/installation-linux.html#ubuntu-kubuntu-xubuntu-lubuntu-saucy-and-above

　　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.