Also allow other toml file readers

[q-wertz]

Currently when the library toml is missing pydocstyle cannot read the pyproject.toml file.

It would be nice if also other toml reader would be supported (tomli), resp. with python 3.11 the built in tomllib library (which is based on tomli as far as I know).

[adamjstewart]

Would be better to migrate to tomli since the tomllib module introduced in Python 3.11 uses the same API.

Supporting multiple readers would increase maintenance overhead since they all have different APIs.