Use pyproject.toml as a version source

[Joshua Casey]

Poetry requires developers to provide a Python version range in the pyproject.toml file.

This field is required [1] and the poetry buildpack will supply this version range to the build plan in paketo-buildpacks/poetry#17.

Since this buildpack does not recognize pyproject.toml as a version source, it will select the default (most recent?) version of python.

• [1] https://python-poetry.org/docs/pyproject/#dependencies-and-dev-dependencies