Drop tasks' is_module_level_function in favor of the established deconstructable paradigm

[Johannes Maron]

The task framework introduces a new is_module_level_function inspection function instead of relying on the established deconstructible paradigm.

Django's task functions (Task.func) need to be serializable. However, Django has an established paradigm for how to do this since the introduction of migrations: deconstructible classes that need to either implement deconstruct or use the @deconstructalbe decorator.

Yes, it will also add support for callables, but more importantly, we don't introduce too many serialization paradigms internally and provide integrators a familiar interface.