Chutney doesn't use python3 if a "python2" binary exists, and fails if it uses python3 anyway.

In the "chutney" shell script, the search list for python binaries is:

binaries="python2 python"

This means that even if python3 is installed as "python", chutney will use python2 instead.

This mistake has led us to accumulate a bunch of python3 incompatibilities.