gh-133530: Modify Heapq docs image settings (gh-133937)

Add class

Author: StanFromIreland

Doc/library/heapq.rst

@@ -315,6 +315,7 @@ The strange invariant above is meant to be an efficient memory representation
 for a tournament.  The numbers below are *k*, not ``a[k]``:
 
 .. figure:: heapq-binary-tree.svg
+   :class: invert-in-dark-mode
    :align: center
    :alt: Example (min-heap) binary tree.