[v7r2] Run most unit tests for Python 3

[chrisburr]

This builds upon #4718 to add support for actually running the unit tests instead of just collecting them. As #4718 hasn't been merged yet, see here for the diff chrisburr/DIRAC@py3-pytest-discover...chrisburr:run-py3-pytest. As usual for these changes, reviewing individual commits might be easier that the entire PR.

There is a lot left to port but I limited myself to the unit tests so we can be reasonably sure I haven't broken anything. I suspect after this is merged it shouldn't take much more to get a functional client installation running with Python 3.

This PR includes #4391 as I didn't want waste time looking for a Python 3 build that uses OpenSSL 1.0.2, @chaen can #4391 be unmarked as WIP now M2Crypto has been updated?

There are three tests which are still disabled as they fail:

• test_BaseType_Unicode and test_nestedStructure fail due to DEncode's string and unicode types being poorly defined. I think this is hard to fix without major changes to DIRAC or backwads incompatible changes to DEncode itself. Hopefully we can ignore it until we've moved to JSON.
• testLockedClass deadlocks frequently due to the use of self.__processThread._Thread__stop(). Officially "threads cannot be destroyed, stopped, suspended, resumed, or interrupted" so the code should be rewritten to follow the supported alternative: "If you want your threads to stop gracefully, make them non-daemonic and use a suitable signalling mechanism such as an Event.".

I also add flaky to the CI as test_cpuUsage struggles to produce enough system CPU usage to be registered thanks performance improvements in Python 3. I think it's worthwhile so we can use it for the handful of other tests which are unreliable.

BEGINRELEASENOTES

*Python 3
NEW: Run most unit tests with Python 3

ENDRELEASENOTES

[andresailer]

I've clicked through all the files.