Support decoding RFC 6979 signatures to (r, s)

This PR will add `pyasn1` as a hard dependency (and removes it from test deps). It adds the ability to encode `(r, s)` to RFC 6979 signatures as well as decoding back to `(r, s)`

Fixes #1285.
I honestly don't know. It makes me uncomfortable to say it doesn't matter, but ASN.1 is explicit about its sequence length. I think this may need to stew for a bit to see what others think.