Since now setuptools supports 2 types of files for configuration
(`setup.cfg` and `pyproject.toml`), it is very trick to provide a single
`read_configuration` function that will provide compatible outputs for
both formats.

Instead the `config.{setupcfg,pyprojecttoml}` modules have their own
`read_configuration` functions that differ between themselves in terms
of arguments and format of the return value.

Therefore the users should be importing directly the specific submodule
and calling the read function from there.

The `config.setupcfg` submodule is advertised as "provisional" in the
deprecation note because the main proposal debated in the setuptools
issue tracker reached some level of consensus around deprecating
`setup.cfg` files.

This will facilitate the implementation of other configuration formats
(such as pyproject.toml as initially defined by PEP 621)