cox_model.ipynb doesn't have formatting run on it (double fmt: off)

[MarcoGorelli]

Notebook title: cox_model.ipynb
Notebook url: https://github.com/pymc-devs/pymc-examples/blob/main/examples/survival_analysis/cox_model.ipynb

Issue description

Cell 4 starts with

# fmt: off

and ends with

#fmt: off

The latter should be

# fmt: on

Note that this issue tracker is about the contents in the notebooks, if
the notebook is instead triggering a bug or error in pymc3, please
report to https://github.com/pymc-devs/pymc3/issues instead

Expected output

If applicable, describe what should happen instead.

Proposed solution

If applicable, explain possible solutions and workarounds.

[OriolAbril]

related to #127 (please tag both issues or only #127 in the PR)

EDIT: I have removed the beginner label because it should be only used in tracker issues to indicate that the notebook should be part of the begginer branch of the documentation tree. I described the tag as "internal use only" because users and new contributors should not care about those labels.

[MarcoGorelli]

Ah, understood, thanks!

[MarcoGorelli]

no longer necessary, as black's built-in notebook support runs cell-by-cell