Bug: setup.py fails to find formatflowed and certifi even though they are installed

setup.py fails for me with Python 3.7 on macOS.
It says:

It looks like you need to install some python modules first
The following packages are required:
- formatflowed
- certifi
You may use your package manager, or run the following command:
pip3 install formatflowed certifi

However the following commands don't report an error:

$ python3 -m certifi
/usr/local/lib/python3.7/site-packages/certifi/cacert.pem
$ python3 -m formatflowed

Furthermore, if I comment out the sys.exit(-1) the tool completes successfully.

Start with setup.py and reproduce the dependency check under Python 3.7 on macOS, using the reported certifi and formatflowed commands as a comparison. Trace why installed modules are reported as missing and verify that setup.py completes without commenting out sys.exit(-1).

Written by the indexing model from the issue text.