Pyyaml: removal of 4.1 broke downstream gates

[prometheanfire]

http://logs.openstack.org/91/578291/2/check/requirements-tox-py27-check-uc/d6c84fe/job-output.txt.gz#_2018-06-29_05_13_33_345222

[prometheanfire]

https://doughellmann.com/blog/2016/02/25/so-youve-released-a-broken-package-to-pypi-what-do-you-do-now/

[penguinolog]

Removal of the package (and distribution files) is a really bad thing. As result, all instances with frozen requirements immediately became broken.

[edmorley]

Same for us:
https://travis-ci.org/mozilla/treeherder/jobs/398044815#L583

[orsinium]

It's broke our locked production requirements too.

[adamcharnock]

Having the same problem here.

[ingydotnet]

I apologize for the fallout on this decision. We were between a rock and a hard place, and we decided that removing a badly broken 4.1 would cause more trouble the longer we waited.

See https://github.com/yaml/pyyaml/issues/193 for details and the path forward to 4.2.