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.

Depends on #20602

CC: @dimpase @videlec @jdemeyer @nbruin

Component: numerical

Issue created by migration from https://trac.sagemath.org/ticket/20657

[mkoeppe]

Dependencies: #20602