Resolve license issue in VIPER

[mbfm]

Before we are able to integrate VIPER fully into openDuT, we need to replace a transitive dependency, which is licensed under LGPL-3.0-only: https://crates.io/crates/malachite

This license is incompatible with our Apache-2.0 license, which would make it impossible to fork the repository under the Apache-2.0 terms.

The transitive dependency is pulled in by RustPython, which we cannot replace, but it is likely possible to change the RustPython code without major changes to make use of a different transitive dependency instead: https://github.com/RustPython/RustPython/issues/5130