libpython: fix Iterable import in pygrass/vector (#734)

Fixes #733

lib/python/pygrass/vector/basic.py

@@ -6,7 +6,7 @@ Created on Tue Jul 31 13:06:20 2012
 """
 import ctypes
 import grass.lib.vector as libvect
-from collections import Iterable
+from collections.abc import Iterable
 
 from grass.pygrass.shell.conversion import dict2html