Remove python-future compatibility code

[knabar]

Similar to ome/omero-py#390, we should review the OMERO.web source code to update all paths with either Python 2 or Python 2/3 logic to pure Python 3+.

[will-moore]

This is just an issue, not a PR, so maybe it shouldn't be linked to a Milestone? I found that a bit confusing since #531 is in the next milestone and is merged, so I think this can be closed?

[knabar]

This is just an issue, not a PR, so maybe it shouldn't be linked to a Milestone? I found that a bit confusing since #531 is in the next milestone and is merged, so I think this can be closed?

Usually I mark all issues with the milestone in which they are (or should be) fixed as well, but have not done so consistently here. We can discuss on Friday if we want to mark issues or not, don't have a strong preference either way.