add python solution for 20
add python solution for 20

lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler,lunixbochs/project-euler