lock: add a workaround for a flufl.lock bug (#2688)

This bug doesn't affect us during regular dvc CLI commands, but is not
visualy pleasing during API usage.

Fixes #2573

dvc/lock.py

@@ -87,6 +87,17 @@ if is_py3:
                     self._tmp_dir, filename + ".lock"
                 )
 
+        # Fix for __del__ bug in flufl.lock [1] which is causing errors on
+        # Python shutdown [2].
+        # [1] https://gitlab.com/warsaw/flufl.lock/issues/7
+        # [2] https://github.com/iterative/dvc/issues/2573
+        def __del__(self):
+            try:
+                if self._owned:
+                    self.finalize()
+            except ImportError:
+                pass
+
 
 else:
     import zc.lockfile