feature: removed the core version as it leads to incompatilibility

When setting up a djangoldp server we have the opportunity to specify an appropriate version. This is messed up if the djangoldp pacakges themself requires a specific version of the core.

I took inspiration of the flask project. There guideline is to include the dependency to flask in the flask enxtension but without managing the version: https://flask.palletsprojects.com/en/1.1.x/extensiondev/#setup-py and an example https://github.com/python-babel/flask-babel/blob/master/setup.py#L24