Motivation
For package dev, a matrix is good, but for a simple project, the .python-version file can just be the source of truth for the test.

Summary of desired enhancement
Provide a flag in usethis ci bitbucket which doesn't create a full test matrix, perhaps named --matrix-python/--no-matrix-python, so that later we could also have --matrix-os/--no-matrix-os and --matrix-dependencies/--no-matrix-dependencies.

If there's no Python matrix, then we will just use the current development version identified for the project.