Incorrect imports in Model

[stranma]

Describe the bug Cannot pass own solver to class Model constructor because of incorrect imports.

To Reproduce To reproduce:

import mip
from mip.cbc import SolverCbc


class MySolver(SolverCbc):
    pass


class MyModel(mip.Model):
    def __init__(self, name: str = "", sense: str = mip.MINIMIZE, *args, **kwargs):
        if kwargs["solver_name"].upper() in ("MYSOLVER"):
            kwargs["solver"] = MySolver(self, name, sense)
        super().__init__(name, sense, *args, **kwargs)


if __name__ == "__main__":
    solver = MyModel(solver_name="MYSOLVER")

  File "C:\nano_sources\shop\shop\models\t_solver.py", line 17, in <module>
    solver = MyModel(solver_name="MYSOLVER")
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...\t_solver.py", line 13, in __init__
    super().__init__(name, sense, *args, **kwargs)
  File "...\Lib\site-packages\mip\model.py", line 107, in __init__
    self.constrs = mip.ConstrList(self)
                   ^^^
UnboundLocalError: cannot access local variable 'mip' where it is not associated with a value

Expected behavior Basically it is not possible to pass any argument in solver and not get the error above. Please see my bugreport for cpython for explanation why this happens and what is wrong

Desktop (please complete the following information):

• Operating System, version: Windows
• Python version: Probably all of them, tested on 3.11 and 3.12
• Python-MIP version (we recommend you to test with the latest version): 1.15

Additional context The reason I want to do this are some inconsistencies in handling nans in LinExpr between CBC and Gurobi. We have created model exploiting the CBC behaviour, which is throwing away anything containing nan. This is useful when defining models with LinExprTensor where some of the elements does not contain any variable, leading e.g. to constraints like 1==1. I wanted to create my own solver to make it consistent.

[stranma]

#389 @sebheger