fix: avoid changing class outside of GIL

Author: henryiii

include/pybind11/iostream.h

@@ -52,9 +52,11 @@ class pythonbuf : public std::streambuf {
 
                 pywrite(line);
                 pyflush();
+
+                // Placed inside gil_scoped_aquire as a mutex to avoid a race
+                setp(pbase(), epptr());
             }
 
-            setp(pbase(), epptr());
         }
         return 0;
     }