[py2py3] Check unittests from Slice 3 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 3,
issue #10114 and PR #10117, do not fail.
A non-exhaustive list of the ut that need to pass is

test/python/WMCore_t/REST_t/Format_t.py
test/python/WMCore_t/Services_t/McM_t/McM_t.py
test/python/WMCore_t/REST_t/Main_t.py

Moreover

• test/python/WMCore_t/WebTools_t/RESTFormat_t.py