secp256r1 is an unsupported EC on linux-s390x

[chenghlee]

Certain tests are failing on linux-s390x because cryptography's OpenSSL backend seemingly does not support SECP256R1 (NIST P-256) on this platform. Running

import cryptography.hazmat.primitives.asymmetric.ec as ec
from cryptography.hazmat.backends.openssl.backend import Backend
backend = Backend()
for alias, curve in ec._CURVE_TYPES.items():
    print(f"{alias}: {backend.elliptic_curve_supported(curve)}")

yields

prime192v1: True
prime256v1: False
secp192r1: True
secp224r1: True
secp256r1: False
secp384r1: True
secp521r1: True
secp256k1: True
sect163k1: True
sect233k1: True
sect283k1: True
sect409k1: True
sect571k1: True
sect163r2: True
sect233r1: True
sect283r1: True
sect409r1: True
sect571r1: True
brainpoolP256r1: True
brainpoolP384r1: True
brainpoolP512r1: True

This affects the 3.3.1 and 3.4.7 packages. It's a little weird since the OpenSSL and PyOpenSSL packages for linux-s390x do support this curve.