Fix travis build error

[guilgautier]

currently errors
https://github.com/guilgautier/DPPy/runs/4382763009

previously passes
https://app.travis-ci.com/github/guilgautier/DPPy/builds/204122410

use github workflow/action instead!

[guilgautier]

CI action with poetry passes https://github.com/guilgautier/DPPy/actions/runs/1526387603 on v1-dev branch 0cf3d23