scripts: fix pylint in dep_tracker.py

BUG=b:234868254
TEST=cros lint

Change-Id: Ieb361e735053233a27038106262e8f9fa0dd9714
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3692191
Tested-by: Sergey Frolov <sfrolov@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Sergey Frolov <sfrolov@google.com>
diff --git a/scripts/dep_tracker.py b/scripts/dep_tracker.py
index 5c6b73f..ced8cb8 100644
--- a/scripts/dep_tracker.py
+++ b/scripts/dep_tracker.py
@@ -85,6 +85,8 @@
     # of processes when jobs is 1 so python exceptions kill the main process,
     # useful for debugging.
     if jobs > 1:
+      # Pool is close()d in DepTracker's destructor.
+      # pylint: disable=consider-using-with
       self._pool = multiprocessing.Pool(jobs)
       self._imap = self._pool.map
     else:
@@ -98,6 +100,10 @@
     self._symlinks = {}
     self._hardlinks = {}
 
+  def __del__(self):
+    """Destructor method to free up self._pool resource."""
+    self._pool.close()
+
   def Init(self):
     """Generates the initial list of files."""
     # First iteration over all the files in root searching for symlinks and