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

Author: sir-sigurd

Doc/library/tempfile.rst

@@ -31,7 +31,7 @@ is recommended to use keyword arguments for clarity.
 
 The module defines the following user-callable items:
 
-.. function:: TemporaryFile(mode='w+b', buffering=None, encoding=None, newline=None, suffix=None, prefix=None, dir=None, *, errors=None)
+.. function:: TemporaryFile(mode='w+b', buffering=-1, encoding=None, newline=None, suffix=None, prefix=None, dir=None, *, errors=None)
 
    Return a :term:`file-like object` that can be used as a temporary storage area.
    The file is created securely, using the same rules as :func:`mkstemp`. It will be destroyed as soon
@@ -72,7 +72,7 @@ The module defines the following user-callable items:
       Added *errors* parameter.
 
 
-.. function:: NamedTemporaryFile(mode='w+b', buffering=None, encoding=None, newline=None, suffix=None, prefix=None, dir=None, delete=True, *, errors=None)
+.. function:: NamedTemporaryFile(mode='w+b', buffering=-1, encoding=None, newline=None, suffix=None, prefix=None, dir=None, delete=True, *, errors=None)
 
    This function operates exactly as :func:`TemporaryFile` does, except that
    the file is guaranteed to have a visible name in the file system (on
@@ -93,7 +93,7 @@ The module defines the following user-callable items:
       Added *errors* parameter.
 
 
-.. function:: SpooledTemporaryFile(max_size=0, mode='w+b', buffering=None, encoding=None, newline=None, suffix=None, prefix=None, dir=None, *, errors=None)
+.. function:: SpooledTemporaryFile(max_size=0, mode='w+b', buffering=-1, encoding=None, newline=None, suffix=None, prefix=None, dir=None, *, errors=None)
 
    This function operates exactly as :func:`TemporaryFile` does, except that
    data is spooled in memory until the file size exceeds *max_size*, or