Checks CI job fails with missing toml dependency

[xadrianzetx]

Black in latest release switched from using toml to tomli. Since toml is no longer installed in Checks workflow, blackdoc will fail the CI job.

Expected behavior

toml should be installed when CI job runs pip install --progress-bar off -U .[checking]

Environment

• Optuna version: 2.9.0.dev0
• Python version: Any
• OS: Any

Error messages, stack traces, or logs

In Checks CI job

Run blackdoc . --check --diff
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.7.11/x64/bin/blackdoc", line 5, in <module>
    from blackdoc.__main__ import main
  File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/blackdoc/__main__.py", line 12, in <module>
    from .blackcompat import (
  File "/opt/hostedtoolcache/Python/3.7.11/x64/lib/python3.7/site-packages/blackdoc/blackcompat.py", line 9, in <module>
    import toml
ModuleNotFoundError: No module named 'toml'

Example

Steps to reproduce

1. Push the code to trigger Checks CI job

[xadrianzetx]

Closing, since blackdoc added toml as dependency in latest release.