test_rebind.py depends on python >=2.7 or >=3.1

It looks like test_rebind.py added as part of legacy/trac#17873 (moved) doesn't work for me on macOS 10.12.6, which has with python 2.6. test_rebind.py uses str.format() without a field name or number, which is new in python 2.7 or 3.1. It also tries to run with python3 (for unknown reasons), but ends up not necessarily doing so because test_rebind.sh explicitly runs $PYTHON or python.

Replacing instances of {} in format strings with {0} seems to work.

What is our minimum required python version anyway?