S256Point.verify and Signature class in ecc.py do not issue warning if "s" value is high

[salmonberry7]

When an ECDSA signature is made with this library using PrivateKey.sign it is canonicalized so the s value is low, ie. so (& see Issue #296) :

s < N // 2

where

N = order of secp256k1 generator point G =
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141

N // 2 = 
7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0        (exact)

However if a signature with high s that was obtained from some other cryptography library (eg. openssl or Python ecdsa package) is verified using S256Point.verify then no warning is issued that the s value is high and needs to be replaced with N - s to be accepted by the Bitcoin network.

Likewise in the Signature class, in the constructor and the parse method, no warning is issued if the s value is high.