DOC/CI: temp pin of decorator (IPython dependency)
The doc builds are failing since yesterday (in enhancingperf.rst, where using the cython ipython magic). By comparing the env from the last working / first failing build on master, one of the differences is a version bump of the `decorator` package, which is a dependency of IPython.

So checking if pinning this package fixes the issue. 
That fixes it, so going to merge this then to get CI green again. Opened an issue about it upstream: https://github.com/micheles/decorator/issues/105 Indeed, this fixes it, so merging, thanks @jorisvandenbossche  @meeseeksdev backport 1.2.x @simonjayhawkins this doesn't need to be backported, as it's already reverted on master