Issue
The official steem-python library depends on the package toml. The package developer removed the latest version (9.3.1). This is an exceptional case as packages should never be removed, a new version should be released. This causes the steem-python library to break and require a manual change after installation.
I have helped quite a few people in the last few weeks install steem-python and had to walk them through manually editing the manifest file to work around this issue.
Solution
The fix is simple, and editing the requirements.txt to use tool version 0.9.3 instead of 0.9.3.1 solves the problem and works perfectly.
Summary
This situation should never happen. It is very bad practice to remove a package from a public package manager. The proper solution would have been to release 0.9.3.2 so third party projects like steem-python do not break. The solution is very simple, just use the previous version.