exclude hash_sizes.py from coverage testing

this is a one-time tool for developers to generate a value table for borg.
the tool is not used at borg runtime.

.coveragerc

@@ -8,6 +8,7 @@ omit =
     */borg/fuse.py
     */borg/support/*
     */borg/testsuite/*
+    */borg/hash_sizes.py
 
 [report]
 exclude_lines =