Limit to compatible Django versions with setup.py's install_requires

[brki]

As per https://packaging.python.org/requirements/#install-requires , CleanerVersion's setup.py should limit installation to known compatible Django versions.

If this has the consequence, though, of downloading a different Django version than the one that a user already has installed, it is not an optimal solution.

Two alternatives (which should perhaps be used together):

• check early in setup.py to see if a non-compatible version of Django is already installed, and error out if that is the case (like in https://trac-hacks.org/ticket/9800#comment:4 ).
• have a runtime check in the top level of versions/models.py, that raises an error if an incompatible version of Django is installed.