[NEW PORT] security/py-securesystemslib: Cryptographic and general-purpose routines

securesystemslib supports public-key and general-purpose cryptography,
such as ECDSA, Ed25519, RSA, SHA256, SHA512, etc. Most of the
cryptographic operations are performed by the cryptography and PyNaCl
libraries, but verification of Ed25519 signatures can be done in pure
Python.

WWW: https://github.com/secure-systems-lab/securesystemslib

[1] https://github.com/secure-systems-lab/securesystemslib/issues/166
[2] https://github.com/secure-systems-lab/securesystemslib/issues/167