vtkmpi4py: empty out Py_{UN,}BLOCK_THREADS

Cython will put _save in a WITH_THREAD block, so it isn't available
without threads. Just having empty code works just as well.

Change-Id: Ia5c63824916a5f14b1d0cf6833e12ed40bf4f71b
Merge topic 'fix-mpi4py-without-threads' into master

0714be97 vtkmpi4py: empty out Py_{UN,}BLOCK_THREADS