replicate scaffold fails for Python template due to missing virtualenv command

[mattt]

On my local dev environment, running the following command produces an error:

$ replicate scaffold --template=python

Error: failed to create virtualenv: failed to run command: exit status 127

Running python -m venv env works for me, though, and I suspect that's more likely to work for other folks, too.

[mattt]

We may also want to gracefully handle environments where Python is accessible from python3 instead of python.