[py2py3] Check unittests from Slice 6 in py3

[mapellidario]

Intro

We need to make sure that the work done in the migration slices
is py3 compatible. In particular, we should make sure that the
unittests that are involved in each slice do not fail.

The schedule for this validation / bug-fixing campaign is kept at #10422

Details

This issue is about checking that the unittests from Slice 6,
issue #10127 and PR #10277, do not fail.
A non-exhaustive list of the ut that need to pass is

test/python/WMComponent_t/DBS3Buffer_t/DBSBufferUtil_t.py
test/python/WMCore_t/ACDC_t/CouchFileset_t.py
test/python/WMCore_t/ACDC_t/CouchService_t.py
test/python/WMCore_t/ACDC_t/CouchCollection_t.py
test/python/WMCore_t/Database_t/MySQLCore_t.py
test/python/WMCore_t/JobSplitting_t/FixedDelay_t.py
test/python/WMCore_t/JobSplitting_t/JobFactory_t.py
test/python/WMCore_t/JobSplitting_t/RunBased_t.py
test/python/WMCore_t/JobSplitting_t/SizeBased_t.py
test/python/WMCore_t/WMBS_t/Job_t.py
test/python/WMCore_t/WMBS_t/Subscription_t.py

We should mae sure that the PR that will close this issue, also solves the problems presented at

• [py2py3] CouchFileset converts file "lfn" to bytes in py3 #10521