style: restore PEP8 blank-line separation after dead-code removal

The deletions in the salvaged commit left some top-level defs/classes
separated by a single blank line. Restore the 2-blank-line separation.

tools/computer_use/cua_backend.py

@@ -76,6 +76,8 @@ _ELEMENT_LINE_RE = re.compile(
 
 def _is_macos() -> bool:
     return sys.platform == "darwin"
+
+
 def cua_driver_binary_available() -> bool:
     """True if `cua-driver` is on $PATH or HERMES_CUA_DRIVER_CMD resolves."""
     return bool(shutil.which(_CUA_DRIVER_CMD))