More fixes for copying a MIP and the MIPVariables created by new_variable

[mkoeppe]

This is a follow-up on #20461.

• a new MixedIntegerLinearProgram.copy method that takes a names keyword argument, enabling this operation:

  sage: p.<x,y> = MixedIntegerLinearProgram()
  sage: q.<newx,newy> = p.copy()
  

  and the less magical syntax

  sage: q, newx, newy = p.copy([x, y])
  

• if MixedIntegerLinearProgram.new_variable has been called, it should set a flag and then if __copy__ (or __deepcopy__) are called, it should display a warning (deprecation??) and refer the user to the new copy method.