Add toml module as explicit requirement

[mgmarino]

black recently dropped this requirement
(psf/black#2301), so this results in an error if
this is run with a newer version of black.

[mgmarino]

Note, we are currently working around this problem in our pre-commit yaml like:

repos:
-   repo: https://github.com/dfm/black_nbconvert
    rev: v0.2.0
    hooks:
    - id: black_nbconvert
      additional_dependencies: [toml]

[dfm]

Thanks! I'll try to publish a release soon!