This introduces BaseUpgradeMethodMutation, which

MoveToDjangoMigrations now subclasses. It handles the core parts of an

upgrade method mutation, and mandates implementation of

generate_dependencies().
A new UpgradeMethodMutator wraps this, applying the mutation and

updating the stored signature when finalizing.
To explain the importance of this mutation, we must talk about how

AppMutator manages mutations and mutators.
AppMutator normally treats mutations that affect models differently

from mutations that don't. AppMutator.run_mutation() will batch

together subsequent model mutations under the same mutator. Each will be

run as a mutation (queueing up operations to modify the database) and

then as a simulation (immediately updating the staging signature). When

collecting SQL, the mutator is finalized, all the operations get turned

into SQL, and a final simulation is run against the final project

signature (which will be stored in the database).
Any mutation that does not apply to models gets mutated and simulated as

well, but immediately. A final simulation is not applied to the final

signature.
While this maybe needs a re-think (we may want to always apply every

mutator at finalization time), short-term, we do need upgrade methods to

be able to modify the final signature at this stage.
This is where UpgradeMethodMutator comes in. If run_mutation() is

passed a subclass of BaseUpgradeMethodMutation, it will be wrapped in

an UpgradeMethodMutator, which will perform a final simulation at

finalization time, ensuring the project signature is correct.
This will be used when applying the upgrade method to move to

evolutions.

All unit tests pass.