bpo-29984: Improve 'heapq' test coverage (GH-992)

diff --git a/Lib/heapq.py b/Lib/heapq.py
index 0e3555c..fabefd8 100644
--- a/Lib/heapq.py
+++ b/Lib/heapq.py
@@ -597,5 +597,5 @@
 
 if __name__ == "__main__":
 
-    import doctest
-    print(doctest.testmod())
+    import doctest # pragma: no cover
+    print(doctest.testmod()) # pragma: no cover