deps: Add missing toml dependency

[david-yz-liu]

Motivation and Context

#807 added a dependency on toml, but this was missing from the setup.cfg package config. I suspect the reason we didn't catch this in testing was one of the dev dependencies has a transitive dependency on toml.

Your Changes

Description: Added toml dependency.

Type of change (select all that apply):

• Bug fix (non-breaking change which fixes an issue)
• Other (please specify): dependency update

Testing

Manual testing.

Checklist

• I have performed a self-review of my own code.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported on Coveralls.
• I have updated the CHANGELOG.md file.