chore: remove unused include from stl.h (#3928)

* change iostream include to ostream include

Author: Skylion007

include/pybind11/stl.h

@@ -13,9 +13,9 @@
 #include "detail/common.h"
 
 #include <deque>
-#include <iostream>
 #include <list>
 #include <map>
+#include <ostream>
 #include <set>
 #include <unordered_map>
 #include <unordered_set>