Clean-up and handling of deprecation warnings for pytest tests

[walesch-yan]

I revisited the pytests for this repository and saw a total of 163 warnings. This PR does some clean up and reduces the number of warnings to 7.
Here is an overview of the changes in this PR:

• Removed the unicode support for python 2.x version
• removed an unused import
• In the configuration files of the mockup version there were two files for beam mockup, one of them unused. Removed that one and removed the deprecated Slits from the other one.
• Move declarative_base import to sqlalchemy.orm

To reduce the total warnings to 7, I will make this a draft and wait on the acceptance of mxcube/mxcubecore#1077 and mxcube/mxcubecore#1072

Quick Note on the remaining 7 warnings:
Those are triggered by passlib using a deprecated api call for pkg_resources. And the flask-security-too package depends on passlib. Unfortunately we are already on the newest version for this package that still supports Python 3.8.

[walesch-yan]

bumped mxcubecore version, to include the changes mentioned in mxcube/mxcubecore#1077 and mxcube/mxcubecore#1072, to now effectively reduce number of warnings to 7