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I also like the patch, but I think a number of changes are
still needed:
- there is no documentation
- there are no test cases
- Don't use Py_BuildValue, use PyString_FromStringAndSize
instead
- Don't stat /dev/urandom when importing os, instead, stat
on first usage, and raise NotImplementedError if no source
of randomness can be found.

As an option, support for PRNG could be added (e.g. by
checking an environment variable), but that can be left to
anybody who actually has and needs PRNG.

As for leaving the fd open: this is ok, I think, if it isn't
opened until first use.