Warn that gc.get_objects() can expose objects under construction #159730

Documentation

The gc.get_objects() documentation does not mention that references created via the function may be partially initialized or allow access to uniquely referenced objects that are not observable by remote threads in any other way.

I'd like like to add a warning like the existing warning in the gc.get_referrers() docs.

For context, this has come up several times:

• Incomplete tuple created by PyTuple_New() and accessed via the GC can trigged a crash #59313 investigated an incomplete tuple whose reference count was changed by a monitoring thread calling gc.get_objects(). The discussion compared this hazard with the existing gc.get_referrers() warning.
• Data race on frozendict reads during construction in the free-threading build #151722, Need to defer GC tracking of frozenset/set construction #152235, and Need to defer GC tracking of array.array construction #153083 reported that gc.get_objects() could expose frozendict, set/frozenset, and array.array objects during construction in free-threaded builds.
• LIST_APPEND and SET_ADD opcodes safety in Free Threading #152288 uses gc.get_objects() to reach a set being built. Its discussion explicitly points to the warning for gc.get_referrers().

Linked PRs

• gh-159730: Warn that gc.get_objects() can expose objects under construction #159731
• [3.14] gh-159730: Warn that gc.get_objects() can expose objects under construction (GH-159731) #159737
• [3.15] gh-159730: Warn that gc.get_objects() can expose objects under construction (GH-159731) #159738