Add the ability to read requirements and extras from pyproject.toml
files, eventually allowing us to move away from requirements.txt files
if we so choose.
Tests are reworked to test this new functionality, with some minor
cleanup to remove unused fixtures.
Change-Id: I3335b5faac72e2e6962d0930eef0e3b704820bbe
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
9b79bc474c