unrpa: use python38

Its dependency uncompyle6 does not support Python 3.9 yet.

pkgs/top-level/all-packages.nix

···
 
   unrtf = callPackage ../tools/text/unrtf { };
 
-  unrpa = with python3Packages; toPythonApplication unrpa;
+  unrpa = with python38Packages; toPythonApplication unrpa;
 
   untex = callPackage ../tools/text/untex { };