Resolve "Create a release using CI/CD"

Closes #17 (closed)

List of changes:

• .gitlab-ci.yml: Before_script is only executed for the tests/linters jobs. They are in the same stage. A new stage (release) is added that is triggered by tagging. The Build job will deploy the package to the Package Registry. The Release job releases the code.
• pyproject.toml: A toml file is added to automatically build the package and write the __version__.py, based on git tagging (using the setuptools_scm module)
• setup.py: the version is removed here
• __init__.py: the version is retrieved here, from the automatically generated __version__.py

\approve @ds @gislars

databaselib/__init__.py

@@ -17,6 +17,11 @@
 # along with this program. If not, see http://www.gnu.org/licenses/.
 
 from .database import AbstractDatabase, SpatiaLiteDatabase, PostGISDatabase
+from importlib.metadata import version, PackageNotFoundError
 
+try:
+    __version__ = version("databaselib")
+except PackageNotFoundError:
+    __version__ = "unknown version"
 
 __all__ = ["AbstractDatabase", "SpatiaLiteDatabase", "PostGISDatabase"]