Hi Ron
It's best to install Python modules for some particular project in a virtual environment. Try this:
brew install pyenv-virtualenv
cd your-project-directory
# create a subdirectory for virtual environment
virtualenv .venv
# activate it in current terminal session
source .venv/bin/activate
# check that we're now using local Python:
which python
python --version
# if you have your project's requirements in a file, install them from it:
pip install -r requirements.txt
# or just install sphinx
pip install sphinx
# now it should work
python -msphinx.ext.intersphinx build/html/objects.inv