Another missing dep for doc building.

tools/circleci_focal_gcc9_heyoka_head_38.sh

@@ -16,7 +16,7 @@ export PATH="$HOME/miniconda/bin:$PATH"
 bash miniconda.sh -b -p $HOME/miniconda
 conda config --add channels conda-forge
 conda config --set channel_priority strict
-conda create -y -q -p $deps_dir python=3.8 git pybind11 numpy mpmath cmake llvmdev boost-cpp mppp sleef fmt spdlog sphinx myst-nb matplotlib pip sympy
+conda create -y -q -p $deps_dir python=3.8 git pybind11 numpy mpmath cmake llvmdev boost-cpp mppp sleef fmt spdlog sphinx myst-nb matplotlib pip sympy scipy
 source activate $deps_dir
 pip install --user sphinx-book-theme