[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.
3.26 (1 February 2018)
======================
New features
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* For ``pyramid`` instance configuration kind, logging is not handled anymore
by CubicWeb but should be configured through ``development.ini`` file
following https://docs.pylonsproject.org/projects/pyramid/en/latest/narr/logging.html.
Backwards incompatible changes
------------------------------
* CubicWebConfiguration method 'shared_dir' got dropped.