Use topological sort for migration operation dependency resolution

[Patryk Zawadzki]

A fixed implementation is provided in ​https://github.com/django/django/pull/3546

The initial implementation was provided by vanschelven, accepted and commited as part of ​https://github.com/django/django/pull/3525. It however broke in Python 3 due to how __eq__ and __hash__ behave.

I opted not to provide a default implementation of deconstruct method to avoid arguments moving between args and kwargs depending on invocation (like the deconstruction of model fields it always produces the canonical form).

[Tim Graham]

I left some cosmetic comments and Carl says it needs to be rebased to apply cleanly and commits squashed.

[Tim Graham <timograham@…>]

In 21e21c7bc2b8bf7ae127e2aa75048a60d05a6e0f:

Fixed #23844 -- Used topological sort for migration operation dependency resolution.

This removes the concept of equality between operations to guarantee
compatilibity with Python 3.

Python 3 requires equality to result in identical object hashes. It's
impossible to implement a unique hash that preserves equality as
operations such as field creation depend on being able to accept
arbitrary dicts that cannot be hashed reliably.

Thanks Klaas van Schelven for the original patch in
13d613f80011852404198dfafd1f09c0c0ea42e6.