MAINT: Add pin for myst-nb on CircleCI config

[skip azp] [skip cirrus]

.circleci/config.yml

@@ -76,6 +76,7 @@ jobs:
           command: |
             pip install numpy cython!=3.0.3 pybind11 pythran ninja meson
             pip install -r doc_requirements.txt
+            pip install "myst-nb<1.0.0"
             pip install mpmath gmpy2 "asv>=0.6.0" click rich-click doit pydevtool pooch threadpoolctl
             # extra benchmark deps
             pip install pyfftw cffi pytest