CI: Fail on Python 3.9 & mpl < 3.6.0
See https://gitlab.esrf.fr/bliss/bliss/-/jobs/482387
- Python 3.7 is the default env of the CI and it works fine
- Python 3.8 and 3.9 are nightly build and fail on Qt test
This is because the last mpl is not used. 3.6.2
There is an unexpected constraints mpl < 3.6.0
on BLISS requirements
This could be removed, or be replaced by != 3.6.0