load get_model when needed

get_model.py

get_model_func = None
def get_model(app, model):
	if get_model_func is None:
		if django is None:
			raise import_failure

		elif django.VERSION[:2] < (1, 7):
		    from django.db.models import loading
		    get_model_func = loading.get_model()

		else:
		    from django import apps as django_apps
		    get_model_func = django_apps.apps.get_model

	get_model_func(app, model)