Consider removing sage as a dependency

[cjpatton]

It was suggested on the CFRG mailing list that VDAF implementers could integrate the reference code directly into their tests, similar to the SUPERCOP project. However such tests would need sage as a dependency, which is not ideal. If we removed the sage dependency, then the only dependency would by python3, which is just about everywhere these days. Of course, they would also need to install pycryptodomex.

I believe we only use sage for the finite field arithmetic and polynomial interpolation, neither of which would be terribly complicated to implement ourselves.

Note that this conflicts with #282.

[cjpatton]

Closed by #445.