yapf formatting fails when pyproject.toml is in the workspace #260

Version: 1.5.0

Steps to reproduce:

• Create workspace with pyproject.toml
• Install pylsp with yapf
• send a format request (must include options)
• error formatting error: toml package is needed for using pyproject.toml as a configuration file

I think I introduced this regression in #134.

Should we add toml as a requirement for yapf formatter like whatthepatch?