Gurobi: Can't access objective

[Markus28]

In get_objective in gurobi.py you are trying to set the const property of LinExpr, which doesn't have a setter.
To reproduce:

>>> import mip
>>> model = mip.Model(solver_name="gurobi")
Set parameter Username
Academic license - for non-commercial use only - expires 2023-07-07
>>> x = model.add_var("x"I)
>>> model.objective = x
>>> model.objective
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mkrimmel/.local/lib/python3.8/site-packages/mip/model.py", line 821, in objective
    return self.solver.get_objective()
  File "/home/mkrimmel/.local/lib/python3.8/site-packages/mip/gurobi.py", line 543, in get_objective
    obj_expr.const = self.get_objective_const()
AttributeError: can't set attribute

Not sure why const cannot have a setter. Either way, you could just use the constructor of LinExpr directly. I may prepare a PR for this because I probably need to have this fixed.

[sebheger]

@Markus28 Feel free to open a PR, I will review it. Don't forget to write a unit-test.

[sebheger]

I guess that could be easily fixed by adding a constant (add_constant) to the expr instead of setting it.