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.
Can you add a comment noting what's invalid about the asn1? (The answer is the last `\x02` is a length, and there's only one byte left... right? You know what's cool, being able to decode ASN.1 by hand...)