bpo-31733: Add PYTHONSHOWREFCOUNT env var (GH-3932)

Add a new PYTHONSHOWREFCOUNT environment variable. In debug mode,
Python now only print the total reference count if PYTHONSHOWREFCOUNT
is set.
diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
index f00f7f6..65988bc 100644
--- a/Doc/using/cmdline.rst
+++ b/Doc/using/cmdline.rst
@@ -663,3 +663,10 @@
 
    If set, Python will print memory allocation statistics every time a new
    object arena is created, and on shutdown.
+
+.. envvar:: PYTHONSHOWREFCOUNT
+
+   If set, Python will print the total reference count when the program
+   finishes or after each statement in the interactive interpreter.
+
+   .. versionadded:: 2.7.15