Remove python-future compatibility code

[sbesson]

See ome/omero-py#390 for the rationale

The list of changes should be identical i.e.:

• all future or past import are replaced by Python 3 calls
• all unnecessary builtins imports are removed
• all Python 2 compatibility logic are removed
• the unmaintained Dockerfile is removed

[sbesson]

Following the release of OMERO.py 5.19.0 (thanks @pwalczysko), the last commits update this PR to also test omero-dropbox on Python 3.12 and update the GitHub actions to Node.js 20

[jburel]

The docker file has been removed. This could be updated. This has been done is other repository e.g. openmicroscopy

[sbesson]

The docker file has been removed. This could be updated. This has been done is other repository e.g. openmicroscopy

I don't mind either way but unlike openmicroscopy I don't think this file was consumed by anything at least in terms of CI. Do you have recommendations guidelines on the target operating system or what the Docker image should be doing?

[jburel]

i am certainly not using it. If anyone using it for testing purpose, we can upgrade otherwise I won't bother
@joshmoore are you using the dockerfile?

[joshmoore]

I assume the last usage was via .travis.yml.

[jburel]

In that case, let's remove it.

[sbesson]

Is this in scope for the upcoming OMERO 5.6.11 release?