Added security/py-ecdsa to the NetBSD packages collection.

py-ecdsa is an easy-to-use implementation of ECDSA cryptography (Elliptic Curve
Digital Signature Algorithm), implemented purely in Python, released under the
MIT license. With this library, you can quickly create keypairs (signing key
and verifying key), sign messages, and verify the signatures. The keys and
signatures are very short, making them easy to handle and incorporate into
other protocols.

(gls)