The short answer is that the latest released version of that package is broken (or rather, depends on a broken package). According to this GitHub issue, the maintainer has committed a fix but not yet released it on PyPI. However, that issue also contains a fix: installing git+https://github.com/python-fedex-devs/python-fedex.git@8d74623123153205d72bb18b774a632ee92972f4 instead of a version from PyPI.
I clicked “Edit requirements.txt directly”, then added that as a line in the file, and voila!
