Hidden dependency on simplejson

The subiss-to-unix script depends on the simplejson package but this is not declared in setup.cfg.

https://git.scc.kit.edu/feudal/regapp-tools/-/blob/master/regapp_tools/bwidmconnection.py#L59

The way I interpret the code, a try: except: wrapped import may be the way to go here instead of declaring a new dependency.