sage behavior of Fq encoding/decoding should be consistent with our Rust code

[redshiftzero]

In #34 in commits 41c0d48 and c1db5ad we updated the Rust encoding/decoding of Fq field elements to ensure that the top three bits of the last byte are unset. We should make sure the Sage code is doing the same in the Decaf_1_1_Point class.