Improve dependency management

[transistortim]

The requirements.txt gets quite large and cluttered if we pin every dependency and every dependency of a dependency of a dependency.

Using https://pypi.org/project/pip-tools/ would make our actual dependencies more clear and reduce the maintenance effort for updating dependencies of dependencies, as this can be done with pip-tools.

[transistortim]

Implemented by https://github.com/svpbde/svpb/commit/56194a3c1821e17feac50956d70479dd96a79d5a, still needs to be documented.