Validate 'Iso8601' is properly defined.

Deliberately exercises the code on 313 of VmomiSupport.py
to validate that the import for datetime parsing is working
as expected.

related to: https://github.com/vmware/pyvmomi/issues/112
python3: fix Iso8601 import statement

The import statement for Iso8601 was not properly
updated. This fix changes the import statement so
it works properly after python3 support changes.

fixes https://github.com/vmware/pyvmomi/issues/112