RuntimeError: glfw version is lower than 3.2.1
my Python Version is 3.6.4,i use pip install glfw,but the glfw version is 3.1.2

```
In [5]: import glfw

In [6]: glfw.get_version_string()
Out[6]: b'3.1.2 X11 GLX clock_gettime /dev/js XI Xf86vm shared'
```

So, glfw's version can't fix the pyxel's.