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Like this for just the set of isoclasses.

def iso(P):
    D = {}
    for x, y in P.relations():
        Q = P.subposet(P.interval(x, y))
        dp = Q.degree_polynomial()
        if dp not in D:
            D[dp] = []
        if not any(Q.is_isomorphic(R) for R in D[dp]):
            D[dp].append(Q)
    return D

then

sage: P = posets.PentagonPoset()
sage: iso(P)
{1: [Finite poset containing 1 elements],
 x + y: [Finite poset containing 2 elements],
 x^2 + 3*x*y + y^2: [Finite poset containing 5 elements],
 x*y + x + y: [Finite poset containing 3 elements]}