Also getting this message today. Catalina 10.15.6
Noted a posting from earlier this year suggesting this was a false positive and would be addressed when virus definitions were updated. The file that is supposedly infected is not one a user can remove, from what I can see. Hopefully it is truly a false positive. I do regular scans and this is the first time have ever seen a MACOS:Pirrit. There was a second one as well today, but the file was some microsoft one in the library and the threat was removed by AVAST.