Drop Python 2 support

[brockho]

With the latest changes in CircleCI and the non-support of Python2 as a language brings us to the point where a single (simple) code breaks some of our code somewhere. As a workaround, we should switch on import between time.clock and time.process_time.

I'll take care of that.

[nikohansen]

For cocopp I think we can drop the support. For cocoex I would try to keep it as long as it is practical to do without painful additional efforts.

[brockho]

With the latest change in the development branch, as discussed today, all current tests run through again. On CircleCI, I removed the python 2 test because it is not anymore supported, but on AppVeyor and Jenkins, we will still have the previous python 2.7 tests for the time being.