Remove unused modulo parameter from Expr.__pow__ and GenExpr.__pow__

[Zeroto521]

Expr.__pow__ and GenExpr.__pow__ in src/pyscipopt/expr.pxi both declared a third parameter named modulo, but neither method body ever reads it. The effect: pow(x, 2, 5) silently ignores the modulus and returns the un-reduced expression, while Python's own pow convention is that a three-argument call is only meaningful for integers and should raise TypeError when the base does not support it.

Removing the parameter makes the behavior match the convention:

>>> x = Model().addVar()
>>> pow(x, 2, 5)
TypeError: Expr.__pow__() takes 3 arguments but 2 were given

x ** 2 and pow(x, 2) are unchanged.

[Copilot]

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[Zeroto521]

Cython maps __pow__ to the nb_power slot, and the generated wrapper is hardcoded to the three-argument signature (self, value, mod=None, /) — verified via inspect.signature, which is independent of whether the source declares two or three parameters. stubtest compares stubs against the compiled runtime signature, so removing mod from the stub necessarily fails with 'stub does not have parameter mod'.