Where is the NoneType located?

You can use type(None) to get the type object, but you want to use isinstance() here, not type() in {...}:

assert isinstance(value, (str, type(None)))

The NoneType object is not otherwise exposed anywhere in Python versions older than 3.10^*.

I’d not use type checking for that at all really, I’d use:

assert value is None or isinstance(value, str)

as None is a singleton (very much on purpose) and NoneType explicitly forbids subclassing anyway:

>>> type(None)() is None
True
>>> class NoneSubclass(type(None)):
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type 'NoneType' is not an acceptable base type

^* As of Python 3.10, you could use types.NoneType, to be consistent with the other singleton types being added to the types module.

Please use type(None). You can use python shell to check like in the below function in which I use type(None) in order to change from None to NoneType.

def to_unicode(value):
'''change value to unicode'''
try:
    if isinstance(value, (str,type(None))):
        return value
    if not isinstance(value, bytes):
        raise TypeError("Expected bytes, unicode, or None; got %r" % type(value))
    return value.decode("utf-8")
except UnicodeDecodeError:
    return repr(value)