In the (C:\Program Files\PostgreSQL\9.6) the "installation-notes.htm" for postgreSQL 9.6 show python 3.3 as being the compatible version. I installed it, put it into my windows 7 path/variables, and can run python 3.3 at the command line. After issuing a
CREATE LANGUAGE plpython3u, I was able to write plpython3u functions. But, QGIS uses python 2.7.5. When I load QGIS, it will no longer load its python console because it explains that python33.dll is not compatible. I suppose I could try reverting to python 2.7, in windows 7, but is that supposed to be compatible with postgreSQL 9.6?
Combining the advice of the two responses, I edited my environmental variables to remove any mention of python3.3. QGis loads, and with the python console! I am not sure whether the python 3.3 environmental variables may be needed in order for plpython3u functions to work in postgreSQL.