Closes#130914: Make graphlib.TopologicalSorter.prepare() idempotent
Relax the rules so that `.prepare()` can be called multiple times, provided that no work has been passed out by `.get_ready()` yet.
the current test depended on integer sets being iterated on in a certain
fixed order. That order is different on PyPy (insertion based) and could
change in CPython in the future in theory. Make the test robust against
a different iteration order by sorting.
The topological sort functionality that was introduced initially in the
functools module has been moved to a new graphlib module to
better accommodate the new tools and keep the original scope of the
functools module.
Daniel Pope