examples/deepzoom: parallelize deepzoom_tile to available CPUs

On Python 3.13+, and on all Python versions on Linux, set the default job
count to the number of available CPUs.  Otherwise, default to 4 jobs as
before.

Signed-off-by: Benjamin Gilbert <[email protected]>

examples/deepzoom/deepzoom_tile.py

@@ -382,6 +382,19 @@ def _shutdown(self) -> None:
 
 
 if __name__ == '__main__':
+    try:
+        # Python 3.13+
+        available_cpus = os.process_cpu_count()  # type: ignore[attr-defined]
+    except AttributeError:
+        try:
+            # Linux
+            available_cpus = len(
+                os.sched_getaffinity(0)  # type: ignore[attr-defined,unused-ignore]
+            )
+        except AttributeError:
+            # default
+            available_cpus = 4
+
     parser = ArgumentParser(usage='%(prog)s [options] <SLIDE>')
     parser.add_argument(
         '-B',
@@ -433,8 +446,8 @@ def _shutdown(self) -> None:
         metavar='COUNT',
         dest='workers',
         type=int,
-        default=4,
-        help='number of worker processes to start [4]',
+        default=available_cpus,
+        help=f'number of worker processes to start [{available_cpus}]',
     )
     parser.add_argument(
         '-o',