SyntaxError on Ubuntu
**Describe the bug**
Trying to install on my ubuntu machine, did compiled GLFW manually, python3 as well. 

Got this:
`pyxeleditor -hTraceback (most recent call last):
  File "/usr/local/bin/pyxeleditor", line 7, in <module>
    from pyxel.editor import run
  File "/usr/local/lib/python3.5/dist-packages/pyxel/__init__.py", line 12
    DEFAULT_BORDER_COLOR: int = constants.DEFAULT_BORDER_COLOR
                        ^
SyntaxError: invalid syntax
`


I also tried in pyenv and got the GLFW wrong version issue

Anyone able to install on Ubuntu or even better, pyenv?



**Desktop (please complete the following information):**
 - Ubuntu
 - python3, python3.7.0


