Return all subclasses of a class in the module.
    If the module defines __all__, only those entries will be searched,
    otherwise all objects not starting with '_' will be searched.

def get_module_plugins(module, classobj):
    """Return all subclasses of a class in the module.
    If the module defines __all__, only those entries will be searched,
    otherwise all objects not starting with '_' will be searched.
    """
    try:
        names = module.__all__
    except AttributeError:
        names = [x for x in vars(module) if not x.startswith('_')]
    for name in names:
        try:
            obj = getattr(module, name)
        except AttributeError:
            continue
        try:
            if issubclass(obj, classobj):
                yield obj
        except TypeError:
            continue