Fix signal-based interrupt on non-main threads

Add a check to detect if the current thread is the main thread before
attempting to set up signal handlers. On Python 3.14+, signal handlers
can only be set up in the main thread of the main interpreter. If not in
the main thread, fall back to non-interruptible execution.

This fixes the test failure in test_interruptible_can_run_function which
was failing with: ValueError: signal only works in main thread of the main
interpreter

Author: kaokab33

src/scancode/interrupt.py

@@ -39,6 +39,8 @@
    traceback and `value` is None.
 """
 
+import threading
+
 
 class TimeoutError(Exception):  # NOQA
     pass
@@ -80,7 +82,11 @@ class TimeoutError(Exception):  # NOQA
     def interruptible(func, args=None, kwargs=None, timeout=DEFAULT_TIMEOUT):
         """
         POSIX, signals-based interruptible runner.
+        Falls back to non-interruptible execution if not in main thread.
         """
+        # Signals only work in the main thread
+        if threading.current_thread() is not threading.main_thread():
+            return NO_ERROR, func(*(args or ()), **(kwargs or {}))
 
         def handler(signum, frame):
             raise TimeoutError