siso: disable linter on staticly inferable check that original_sigint_handler is callable.

Change-Id: I6429a0f91a52449438938e4c82df51e96a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/7259440
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Alex Ovsienko <ovsienko@google.com>

siso.py

@@ -456,7 +456,7 @@ def main(args, telemetry_cfg: Optional[build_telemetry.Config] = None):
     def _ignore(signum, frame):
         try:
             # Call the original signal handler.
-            original_sigint_handler(signum, frame)
+            original_sigint_handler(signum, frame)  # type: ignore
         except KeyboardInterrupt:
             # Do not reraise KeyboardInterrupt so as to not kill siso too early.
             pass