Update pyyaml version to >=4.2b1 for CVE-2017-18342

[bitdivision]

What? and Why?

Github is warning about a CVE on all repos which depend on sdc-cryptography. This updates pyyaml to avoid the issue.

Checklist

• CHANGELOG.md updated? (if required)

[bitdivision]

4.2b1 is a prerelease and PyYAML have not released a new version to fix this CVE. This is an old security issue and is noted in the documentation.

Development on 4.2 seems to have stalled in July 2017, there hasn't been another release since then. The original fix was reverted. Some discussion of the github reporting here: yaml/pyyaml#243 and the underlying issue / release plan here: yaml/pyyaml#193

We do not use the load method in this project, so the original CVE does not apply.

I would suggest that the reporting is removed by github, since there is no released version to fix it.

Closing for now, feel free to reopen if you disagree.