Use python 3.8 for building docs

[mslw]

Added a .readthedocs.yaml file to make readthedocs use Python 3.8 (instead of 3.7) when building documentation. On 3.7 (default) the build failed when trying to install the package dependencies due to version mismatch (pandas / numpy).

This should cause the docs to start building again. Fixes #98

[mslw]

Interestingly the PR triggered a docs build (but only for version:stable) which used 3.7 and failed with the same error as previously, as if nothing changed. If the file was incorrect, readthedocs would complain:

Read the Docs validates every configuration file. Any configuration option that isn’t supported will make the build fail. This is to avoid typos and provide feedback on invalid configurations.

Could it be that we need to merge it and then see if it works (and revert if it doesn't)?

[mih]

This might be, let's try. The file looks good to me.

[mih]

So worked! The stable build would start passing, as soon as we make a release of this extension.