Python3: allow most tests to run with Python3 with small changes

As a first step to allowing our python test suite to run with Python3, we should include a set of changes that:

1. continue to work with Python2, running all tests
2. with possible minor alterations, can run most of the test suite under Python3

Obviously, an ideal final goal is to be able to run and pass all tests under both Python2 and Python3 with a single code base.  But it will be easier to get there if we initially incorporate changes that work with these relaxed rules.  Then we can tackle the remaining goals in later tickets.

This ticket is a first step to fully solving WT-3929.