update getLumiListInValidFiles() in UserUtilities

[belforte]

Current code only works in python2, and even in that case fails with new go-based DBS server.
See
https://cms-talk.web.cern.ch/t/error-when-using-getlumilistinvalidfiles/16240/3

• quick fix: make old client work with new server using the tip from Kifri [1]
• add a warning in case it is called in python2 script.

long term fix: use dasgoclient - likely not worth !

[1]
I edited the following lines:

CRABClient/src/python/CRABClient/UserUtilities.py

Lines 175 to 177 in 7248a05

	lumis = f['lumi_section_num']
	for lumi in lumis:
	runLumiPairs.append((run,lumi))

to

lumi = f['lumi_section_num']
runLumiPairs.append((run, lumi))

since f['lumi_section_num'] is just an int. It now works as expected.

[belforte]

But in reality new python3 DBS Client is not part of CMSSW distribution. It needs to be pip-installed by users.
So I will go for dasgoclient