Address newly failing modexp tests as of ethereum/tests `v13.1`

New modexp tests included in *ethereum/tests* `v13.1` are failing and need addressing. Tests located [here](https://github.com/ethereum/py-evm/blob/0ac74f65f04062f44f27e8d02cec8aeb11e31671/tests/json-fixtures/blockchain/test_blockchain.py#L1175-L1180).