Add another python27 exception on the docs

doc/nitpick-exceptions.txt

@@ -18,6 +18,7 @@ cpp:type bob::io::base::HDF5File
 # exceptions are not found in python 2.7
 py:exc RuntimeError
 py:class tuple
+c:func PyUnicode_FSConverter
 
 # these don't exists on numpy's manual
 py:class numpy.uint8