Darlog-py23

- String module: `str_types` + use more specific `UnicodeError`
- Switch to a declarative `setup.cfg` + cleanup the build files + document them with comments for the future

- major change: `darlog_py23` -> `darlog.py23` (with namespace)
- update `setuptools` config

fix: fallback signature for `attrs`

* Python version constants.
* string: `str_format`, `unicode`
* `dataclass_fallback`, used instead of dummy decorators as the last resort for `dataclass` and `attrs`.

Use `darlog-pypi`; cross-links between pypi and github.
No effect on functionality.

Remove unnecessary dependency from `six` package.