Iris Read The Docs build is failing due to dependencies

[tkknight]

The Read The Docs (RTD) build using a conda yaml file for the env definition to use. A build is triggered on every update to Iris master. The yaml file:

• https://github.com/SciTools/iris/blob/master/ci/requirements/readthedocs.yml)

This is separate from the Iris requirements:

• https://github.com/SciTools/iris/tree/master/requirements)

Due to recent issues with dependencies and then some being being pinned (#3759, #3758, #3757 ) only in the Iris requirements - the RTD build is now failing (see https://readthedocs.org/projects/scitools-iris/builds/). Don't panic this is ok for now, as RTD shows the latest successful documentation build - it will only update the documentation upon the next successful build.

Once the current dependencies issues have been resoled and unpinned, the RTD yml file can also be updated.

An overhaul of the Iris requirements is on the horizon, perhaps we could align somehow to the RTD yaml file.