Django 3.0.1 support

[daneb255]

In Django 3.0.1 there is a DeprecationWarning with the imp module in the env.py file.

django_assets\env.py:1: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
import imp

I would be very happy about a fix :)

Greetings

[daneb255]

Is it fixed ? :)