feat: add a Dunder Version and use it in setup.cfg

The version string was already defined in setup.cfg, but there it is only available to the build and packaging mechanism, not to the Python runtime. That would typically be made possible by the so called "Dunder Version" which is a __version__ attribute.

The Python Packaging User Guide describes ways to use a single source for both. Accessing the "Dunder Version" from setup.cfg is one.

Now you can do:

>>> import desyrdl
>>> print(desyrdl.__version__)
1.0.1