security/py-ecdsa: Update to 0.16.0

• Update to 0.16.0
• Add test target
• Pet portlint

Changelog:

https://github.com/warner/python-ecdsa/releases/tag/python-ecdsa-0.16.0

Reviewed by: koobs
Approved by: koobs (ports, mentor, maintainer)
Differential Revision: https://reviews.freebsd.org/D27187
MFH: No (API changes, unclear on compatibility)