[pyramid/security] use cryptographically secure random generator
Based on django source code in case SystemRandom is not available.
According to python documentation:
https://docs.python.org/2/library/random.html
> Warning
> The pseudo-random generators of this module should not be used for security
> purposes. Use os.urandom() or SystemRandom if you require a cryptographically
> secure pseudo-random number generator.
.. -*- coding: utf-8 -*-
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