model: Initialize _agents attribute on __new__

[rht]

This is to avoid the warning:

/projectmesa/mesa-examples/examples/boltzmann_wealth_model_experimental/model.py:53: FutureWarning: The Mesa Model class wasn’t initialized. In the future, you need to explicitly initialize the Model by calling super().__init__() on initialization.
  super().__init__(unique_id, model)

So that users won't have to call super().__init__() in their model code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (811b605) 79.96% compared to head (4b84a55) 79.96%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1937   +/-   ##
=======================================
  Coverage   79.96%   79.96%           
=======================================
  Files          15       15           
  Lines        1143     1143           
  Branches      245      245           
=======================================
  Hits          914      914           
  Misses        199      199           
  Partials       30       30           

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[EwoutH]

Looks useful, thanks! I don’t have the Python expertise to judge the code changes, but high-level, I approve!

---

Not necessary for this PR, but maybe we should also consider on a broader spectrum if we want to require super().__init__() calls on initializing a subclass of the Agent or the Model. Can’t we just move everything to __new__? I started a discussion on this here:

• Requiring `super().__init__()` in subclass initialization of Agent and Model #1938

[rht]

Closing because self._agents is attribute initialization, which has to be located in __init__. No exception.