bpo-33239: Fix default value of 'buffering' parameter in docs of tempfile.* functions (GH-21763)

`None` doesn't work:

```python
>>> import tempfile
>>> tempfile.TemporaryFile(buffering=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sergey/tmp/cpython-dev/Lib/tempfile.py", line 607, in TemporaryFile
    return _io.open(fd, mode, buffering=buffering,
TypeError: 'NoneType' object cannot be interpreted as an integer
```

Automerge-Triggered-By: @vsajip