Cleanup and align code after the Python3 transition

[belforte]

With v3.211130 we have a single CRABClient code base which works with all CMSSW versions, either with python2 or python3 interpreter.
We need to cleanup things.

• make the crab-dev variant print a message discouraging users from using it warn users of crab-dev #5134
• remove old branches
• take proper care of Replace current configuration validation with a py3 compatible one #5004
• update documentation, see also Adapt documentation to latest client #5124
• move also crab-pre to this version (in January?) and finally remove WMCore from CRABClient builds
• Stop distributing py2 version of DBS Client inside CRAB, given now we have a pip-installable py3 client. There is no need to run py2 DBSClient inside CRABClient and users who need it in their scripts should switch to new client anyhow. stop distributing ol python2 DBS Client #5182
• the only reference to DBS Client in CRABClient is

  CRABClient/src/python/CRABClient/UserUtilities.py

  Line 144 in b610228

  def getLumiListInValidFiles(dataset, dbsurl = 'phys03'):

  which is offered as a utility for users but not used in the client code. update getLumiListInValidFiles() in UserUtilities  #5175
• review code and look for other places where cleanup is needed
• and finally, improve code quality and robustness in py2/3 compatibility : replace scattered ad-hoc py2/3 compatibility fixes with solid ones  #5139

@ddaina feel free to have a look and add to the list thing which I forgot. Of course cleanup is sort of a never ending work, we don't need to be perfect here, simply take care of things which will be tricky once we have forgot details, but can always do more cleanup as we hit thing.