Bug with matplotlib 3.8

**Code for reproduction**
https://py-eddy-tracker.readthedocs.io/en/latest/python_module/16_network/pet_ioannou_2017_case.html#local-radius-timeline

**Actual outcome**

x is in datetime64 and directly interpreted by matplotlib with 3.8 version

And Detection doesn't work because contour object change in 3.8