streamline toml libraries

[pmeier]

#593 introduced a runtime dependency on tomli, while we have been using tomlkit. For the fairly basic usage that we have, there is only one important difference: tomlkit can read and write TOML files, while tomli can only read them. Since we also need the functionality to write config files, we currently can't remove the tomlkit dependency.

However, that might change at runtime with #449 and #450. If we remove the functionality of creating a config file from the object, we can remove tomlkit as runtime dependency. We still have to have it as dev dependency though, as we use it in our script to update the optional dependencies.