fix new non-API NOTE

[t-kalinowski]

Version: 1.44.0
Check: compiled code
Result: NOTE 
  File ‘reticulate/libs/reticulate.so’:
    Found non-API call to R: ‘SET_OBJECT’
  
  Compiled code should not call non-API entry points in R.
  
  See ‘Writing portable packages’ in the ‘Writing R Extensions’ manual,
  and section ‘Moving into C API compliance’ for issues with the use of
  non-API entry points.

Flavors: r-devel-linux-x86_64-debian-clang, r-devel-linux-x86_64-debian-gcc, r-devel-linux-x86_64-fedora-clang, r-devel-linux-x86_64-fedora-gcc