add note about PyGSL PyPI release

.github/workflows/ode-toolbox-build.yml

@@ -73,7 +73,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip pytest pycodestyle codecov pytest-cov wheel
           python -m pip install numpy
-          if [ "${{ matrix.with_gsl }}" == "1" ]; then python3 -m pip install -v https://github.com/pygsl/pygsl/archive/refs/tags/v2.4.1.tar.gz ; fi
+          if [ "${{ matrix.with_gsl }}" == "1" ]; then python3 -m pip install -v https://github.com/pygsl/pygsl/archive/refs/tags/v2.4.1.tar.gz ; fi # this should be "pip install pygsl", but see https://github.com/pygsl/pygsl/issues/59
           python -m pip install -r requirements.txt
           export PYTHON_VERSION=`python -c "import sys; print('.'.join(map(str, [sys.version_info.major, sys.version_info.minor])))"`
           echo "Python version detected:"