title: kivy and python3 doesnt work on ubuntu 18.04
body: <p>I have laptop with ubuntu 18.04 and i use for programming in python3. I decided to learn how i can make GUI and i chosen Kivy because of its multi-platform feature. After adding Kivy ppa and installing python3-kivy package with apt i tried to test the simple example with one button in the first page of kivy.org
  but it return this errors:</p>
  
  <p>screenshot from my terminal</p>
  
  <p><img src="https://i.stack.imgur.com/Jpy2U.png" alt=""></p>
  
  <p>after that i installed python-kivy package for python2 and run this test file again but this time there is no problem and the GUI showed correctly. </p>
  
  <p>the program runs correctly</p>
  
  <p><img src="https://i.stack.imgur.com/GtTzv.png" alt=""></p>
  
  <p>now i can use python2 but i always use python3 before and i want to use it with kivy. Can anyone tell me why this problem happens?</p>