Support newer versions of Ruamel

Description

Support newer version of the ruamel-yaml library.

Changes proposed in this merge request:

• Fix one line of code that is incompatible with newer ruamel versions.
• Bump ruamel-yaml requirement to >= 0.15.52 so that tests run with a newer ruamel version.

Note that the change is backwards-compatible, and versions of ruamel less than 0.15.52 will still work

This merge request, when approved, will close: #790 (closed)

changed the description

mentioned in issue #790 (closed)

Thanks @kerrick1 for the patch !

Unfortunately, I don't think consuming a newer version of ruamel.yaml is appropriate for us until the new API stablizes.

See the following statement from the ruamel.yaml PyPI page:

Starting with version 0.15.0 the way YAML files are loaded and dumped is changing. See the API doc for details. Currently existing functionality will throw a warning before being changed/removed. For production systems you should pin the version being used with ruamel.yaml<=0.15. There might be bug fixes in the 0.14 series, but new functionality is likely only to be available via the new API.

I think the prudent thing to do is to follow the upstream recommendation instead, as this patch would allow consumption of further updates which are not guaranteed to be stable.

@kerrick1 - with the above comment in mind I am going to close this MR for now: we can re-open it once the ruamel API is stable

closed