fix: avoid changing class outside of GIL

Author: henryiii

include/pybind11/iostream.h

@@ -52,9 +52,12 @@ class pythonbuf : public std::streambuf {
 
                 pywrite(line);
                 pyflush();
+
+                // Placed inside gil_scoped_aquire as a makeshift mutex
+                // to reduce MSVC issues
+                setp(pbase(), epptr());
             }
 
-            setp(pbase(), epptr());
         }
         return 0;
     }
@@ -70,6 +73,8 @@ class pythonbuf : public std::streambuf {
           d_buffer(new char[buf_size]),
           pywrite(pyostream.attr("write")),
           pyflush(pyostream.attr("flush")) {
+        // This is a makeshift mutex to reduce MSVC issues
+        gil_scoped_acquire tmp;
         setp(d_buffer.get(), d_buffer.get() + buf_size - 1);
     }