Python bindings require virtual environment

The Python bindings are currently silently disabled if no virtual environment package is found on the system (https://gitlab.dune-project.org/core/dune-common/-/blob/master/cmake/modules/DunePythonVirtualenv.cmake#L129). While it's good practice to use a virtual environment, I don't see why this should be a hard requirement for using the Python bindings.

It might be that it's currently required (although this is not obvious without having an overview over all CMake files). In this case, this issue should be considered a feature request.

If a virtual environment stays/is a hard requirement, this should be checked earlier IMO, e.g. here.