Upsilonconf

Added
* Introduce `PlainConfiguration` class for more pythonic mutable configurations.
* Introduce `FrozenConfiguration` class for immutable configurations.
* Introduce `ConfigurationBase` interface for custom configurations.
* Introduce flat views to enable iteration over flattened configurations.
* Add option to lo
* Write proper [documentation](https://hoedt.github.io/upsilonconf).

Changed
* Rename `Configuration` to `CarefulConfiguration`.
* Remove discrimination between attributes and keys in `CarefulConfiguration`.
* Issue warnings instead of errors for keys that are no valid attribute names `CarefulConfiguration`.
* The default configuration object when reading configuration files is `PlainConfiguration` instead of `CarefulConfiguration`.
* Raise error when using empty or nested tuples as index to configuration objects.
* `io.FlexibleIO` has been deprecated in favour of `io.ExtensionIO`.
* Use UTF-8 encoding when reading and writing configuration files.

Added
* Add support for reading/writing TOML config files.
* Add functionality to get flat version of configuration.

Changed
* Make `overwrite` method preserve original ordering.
* Fix type hints (and code) so that `mypy` does not find any errors.

Changed
* fix type hints for `upsilconf.io.load_config` and `upsilonconf.io.save_config`
* use proper implementation for JSON default encoding.

Added
* Feature to allow key modifications during loading and saving configs sphh (3)
* Feature to convert configs to/from free-form dictionaries sphh (6)

Changed
* Refactoring of `io` submodule to allow greater flexibility (with lazy loading) sphh (4)
* Wildcard import `from upsilonconf import *` now imports `load_config`, `save_config` and `config_from_cli` (instead of `load`, `save` and `from_cli`) to reduce chances of conflicting names.

First proper release to PyPI.

Fixed import issues and exposed OptionalDependencyError