toConvert = pd.DataFrame({
    'x': [1, 2],
    'y': [10, 20],
    'z': [(101, 102, 103), (201, 202)]
})

def flatten(df):
    tmp = []
    def backend(r):
        x = r['x']
        y = r['y']
        zz = r['z']
        for z in zz:
            tmp.append({'x': x, 'y': y, 'z': z})
    df.apply(backend, axis=1)
    return pd.DataFrame(tmp)

print(flatten(toConvert).to_string(index=False))

x   y    z
1  10  101
1  10  102
1  10  103
2  20  201
2  20  202