Cache Model._meta.swapped

[Adam Johnson]

Another candidate for caching, like #35232 before.

The Model._meta.swapped property returns the model that this one has been swapped for. Since most models are not swappable (only auth.User is officially swappable), it returns None in nearly all cases.

I found this property was the most called function in Django when profiling a subset of Django’s tests, with:

$ python -m cProfile -o profile runtests.py --parallel 1 *model*

$ python -m pstats profile <<< 'sort ncalls
stats 10000' | less

This showed 439,484 calls to swapped, taking 29ms of the 5.597s test run, or 0.5% of the total runtime.

After adding @cached_property, this is reduced to 3,653 calls, rounding down to 0ms.

[Natalia Bidart]

Thank you Adam! I a little on the fence on this one but let's give it a chance. Could you please a test in the PR to cover for the new (non trivial) logic?