Support verification of ed25519 sigs in pure python, by default
Support verification of ed25519 sigs in pure python, by default

Importing keys.py shouldn't raise ImportError for missing crypto libraries

Signed-off-by: Vladimir Diaz <2e5c68b2aca5fc3a1efbd1c951253c7eb2061750@gmail.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib