fix: avoid changing class outside of GIL

include/pybind11/iostream.h

@@ -52,10 +52,12 @@ private:
 
 
                 pywrite(line);
                 pywrite(line);
                 pyflush();
                 pyflush();
-            }
 
 
+                // Placed inside gil_scoped_aquire as a mutex to avoid a race
                 setp(pbase(), epptr());
                 setp(pbase(), epptr());
             }
             }
+
+        }
         return 0;
         return 0;
     }
     }