How to prevent debhelper using Python 2

[Thomas Kluyver <takowl@gmail.com>]

While working on the python3-sympy package, I've seen that if Python 2 is installed, various dh_* commands, like dh_auto_clean, will automatically try to run setup.py in Python 2. In this case, setup.py checks the Python version and fails. This isn't a problem when building in a clean environment like a buildd, but it's annoying when trying to build the package locally.

I could specify the relevant override_dh_* targets in debian/rules, but I mostly want to let the default things run, just not with Python 2. Is there a good way to specify this in the packaging?

Thanks,
Thomas