Django 1.11 support on python 3

[AgDude]

The way that django-eventstream requires channels in setup.py prevents dependency resolution for django 1.11 on python 3. I am upgrading a project which uses pip-tools for dependency management and get the following output:

There are incompatible versions in the resolved dependencies:
  Django<2 (from -r requirements/requirements.in (line 33))
  Django>=2.2 (from channels==2.4.0->django-eventstream==3.1.0->-r requirements/requirements.in (line 34))

pip-tools does not currently have an option to override that. jazzband/pip-tools#561

The way this is defined effectively makes django-eventstream only compatible with django>=2.2 on python 3. Since pip doesn't have strict dependency resolution this probably goes unnoticed in most cases.

I would be happy to offer a PR, but I don't see a good solution without forcing users to explicitly install channels, which would likely break things for many users. It seems like this might be addressed with extra_requires but I am not seeing how to make it dependent on the django version.

[jkarneges]

It looks like extra_requires could be used to specify channels as an optional dependency, that would only be installed if the user asks for it (for example by specifying a dependency to django-eventstream[channels]). This would be a breaking change though, and ideally the option would be inverted (channels enabled by default).

How about copying the django-eventstream module into your source tree, so you can control the dependencies and not attempt to install channels?

[jkarneges]

I've decided to simply require installing channels manually, to make it easier for users to select between older Django versions and the latest Channels versions. The major version of django-eventstream has been bumped to avoid surprises. Hopefully this fixes your issue.