fix: avoid cell magic in colab cell

[redeboer]

Follow-up to #250. The cell magic conditionals didn't work well, which slowed down CI.

[redeboer]

There's still another bug related to the custom hook (which create this colab cell): they do not work in the tox -e sty job. So eventually I want to move these hooks out again from the repo and put them under a separate ComPWA/pre-commit-hooks repository (which allows you to install the hook as a repo hook with python as language).
https://pre-commit.com/#new-hooks

[codecov]

Codecov Report

Merging #251 (e91bb8d) into master (1e36bfc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   77.70%   77.70%           
=======================================
  Files          13       13           
  Lines         619      619           
  Branches       92       92           
=======================================
  Hits          481      481           
  Misses        103      103           
  Partials       35       35           

Flag	Coverage Δ
unittests	77.70% <ø> (ø)

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