makemigrations creates unneeded migrations

[smcoll]

The makemigrations management command will create a new migration for the model inheriting SignatureType whenever the keys in settings.ANYSIGN['BACKENDS'] change. This can happen if the setting has different values in different environments. makemigrations will bake the value derived from the first environment into a migration, and then migrate in the other environment will think the migrations are out of sync with the models.