Allow PyMC >= 5.15

[AlexanderFengler]

Right now we are capping the PyMC dependency version at <5.15. Simply releasing the version constraint fails our tests, because some class signatures changed on the PyMC side.

This is a placeholder issue for the internal work necessary to accommodate PyMC >=5.15.

[digicosmos86]

I'll work on this now. Graphing will still break, which will be fixed once bambi is updated