Update setup.py for channels==4.2.0 usage

[westhomas]

The setup.py is using channels~=4.0.0 which is preventing it from allowing channels >=4.0.0. i.e. 4.2.0 is not allowed:

ERROR: Cannot install channels-postgres==1.0.5 and channels==4.2.0 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested channels==4.2.0
    channels-postgres 1.0.5 depends on channels~=4.0.0

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip to attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts

Would it be possible to make the setup.py more lenient? Or is there some incompatibility that prevents usage?

[cro]

I just opened a PR for this.

[danidee10]

version 1.0.6 has been published to pypi

Thanks! @cro