replace _version file

[egpbos]

In #260 we tried removing the _version file, as the idea was that it would be generated automatically by setuptools_scm. However, it seems like this feature was removed since 8.0.0 (see pypa/setuptools-scm#960). We should find another way to store the version in the package.

[egpbos]

The Python template recommends using bump-my-version for this, by the way. Configuration for this can be found in e.g. https://github.com/NLeSC/python-template/blob/0ff2ed6a9ff4cf024085d02d4d95502e06554eeb/template/pyproject.toml.jinja