Some modules and methods have changed names in Python 3.
The `thread` module has been renamed to `_thread` and the old name
gives error when run on Python 3:

```python
Traceback:
/opt/slapgrid/b0df76c24a1d2728ccf3e276f07c1790/parts/python3/lib/python3.9/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
wcfs/client/client_test.py:32: in <module>
    from wendelin.wcfs.wcfs_test import tDB, tAt, timeout, eprint
wcfs/wcfs_test.py:44: in <module>
    from thread import get_ident as gettid
E   ModuleNotFoundError: No module named 'thread'
```

In a similar vein, the `items` method of dictionaries plays the same
role as the old `iteritems`.

We use the `six` module to paper over these differences.

/reviewed-by @kirr
/reviewed-on nexedi/wendelin.core!27