test: improve Python3 compatibility for swift_build_sdk_interfaces

Return a "unicode" "string" (UTF-8 decoded data) rather than a
bytes-like object as the rest of the script expects the normal unicode
string.

utils/swift_build_sdk_interfaces.py

@@ -105,7 +105,7 @@ def run_command(args, dry_run):
     try:
         out, err = proc.communicate()
         exitcode = proc.returncode
-        return (exitcode, out, err)
+        return (exitcode, out.decode('utf-8'), err.decode('utf-8'))
     except KeyboardInterrupt:
         proc.terminate()
         raise