Improve how python backend returns values

[daniel-sintef]

Currently cast the AttributeDict.json() return values in the python backend to bytes manually to make it match the services backend exactly. E.g.,

otelib/backends/python/base.py: return bytes(AttrDict(**session_update).json(), encoding="utf-8")

However, this can probably be improved. Possibly by adding a new bytes return option to AttrDict.

[CasperWA]

I think the solution mentioned in #86 makes the most sense. I have also implemented a solution for this in #134. @daniel-sintef feel free to close this issue if you agree.