Linux Debian module pixel init error
Error import main pyxel module.

```
>> from pyxel.examples import install
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/panci/.local/lib/python3.5/site-packages/pyxel/__init__.py", line 12
    DEFAULT_BORDER_COLOR:int = constants.DEFAULT_BORDER_COLOR
```

Python version

```
Python 3.5.3 (default, Sep 27 2018, 17:25:39) 
[GCC 6.3.0 20170516] on linux
```

I installed the package using pip3

`pip3  install --user  pyxel`

Debian Stretch
Python 3.5.3

