[BEAM-5620] rename assertItemsEqual to assertCountEqual for PY3 compatibility

[Fematich]

Replaced all assertItemsEqual by assigning it to assertCountEqual on Python 2, as discussed in BEAM-5620 and illustrated in

beam/sdks/python/apache_beam/coders/coders_test_common.py

Line 61 in c34c367

if sys.version_info[0] < 3:

I have added def setUpClass(cls) if it wasn't available (only setUp was in most cases), so that it shouldn't be rerun for every test of the class.

This is is part of a series of PRs with goal to make Apache Beam PY3 compatible. The proposal with the outlined approach has been documented here: https://s.apache.org/beam-python-3

@tvalentyn @manuzhang @charlesccychen @aaltay @splovyt @Juta

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

[tvalentyn]

LGTM. Thank you, @Fematich.