python: make copy a dataflow step

Author: yoff

python/ql/lib/semmle/python/frameworks/Stdlib.qll

@@ -3973,7 +3973,7 @@ private module StdlibPrivate {
       or
       input = "Argument[self]" and
       output = "ReturnValue" and
-      preservesValue = false
+      preservesValue = true
     }
   }