fixup! style: clang-tidy: default checks and fix bug in iostream deconstruction

Author: henryiii

include/pybind11/iostream.h

@@ -38,8 +38,7 @@ class pythonbuf : public std::streambuf {
         return sync() == 0 ? traits_type::not_eof(c) : traits_type::eof();
     }
 
-    // This function must be non-virtual to be called in a destructor
-    int _sync() {
+    int sync() override {
         if (pbase() != pptr()) {
             // This subtraction cannot be negative, so dropping the sign
             str line(pbase(), static_cast<size_t>(pptr() - pbase()));
@@ -55,10 +54,6 @@ class pythonbuf : public std::streambuf {
         return 0;
     }
 
-    int sync() override {
-        return _sync();
-    }
-
 public:
 
     pythonbuf(object pyostream, size_t buffer_size = 1024)
@@ -73,7 +68,9 @@ class pythonbuf : public std::streambuf {
 
     /// Sync before destroy
     ~pythonbuf() override {
-        _sync();
+        // Must be fully qualified, since the vtable for the base is loaded
+        // here, not the child.
+        pythonbuf::sync();
     }
 };