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

```
/pybind11/include/pybind11/iostream.h:71:9: warning: Call to virtual method 'pythonbuf::sync' during destruction bypasses virtual dispatch [clang-analyzer-optin.cplusplus.VirtualCall]
        sync();
        ^
/pybind11/tests/test_iostream.cpp:72:5: note: Calling '~scoped_ostream_redirect'
    });
```