Subclassing typing.NamedTuple an inheriting from a mixin class does not work. It does work for collections.namedtuple, and can be worked around by modifying typing.NamedTupleMeta:

>>> import collections
>>> import typing
>>>
>>>
>>> class Mixin:
...     def mixin(self):
...         return "mixin"
...
>>>
>>> class CollectionsNamedTuple(Mixin, collections.namedtuple('CollectionsNamedTuple', [
...     "a",
...     "b",
... ])):
...     pass
...
>>>
>>> class TypingNamedTuple(Mixin, typing.NamedTuple):
...     a: str
...     b: str
...
>>>
>>> class NamedTupleMeta(typing.NamedTupleMeta):
...     def __new__(cls, typename, bases, ns):
...         cls_obj = super().__new__(cls, typename + '_nm_base', bases, ns)
...         bases = bases + (cls_obj,)
...         return type(typename, bases, {})
...
>>>
>>> class FixedTypingNamedTuple(Mixin, metaclass=NamedTupleMeta):
...     a: str
...     b: str
...
>>>
>>> cnt = CollectionsNamedTuple("av", "bv")
>>> tnt = TypingNamedTuple("av", "bv")
>>> ftnt = FixedTypingNamedTuple("av", "bv")
>>>
>>> cnt.mixin()
'mixin'
>>> ftnt.mixin()
'mixin'
>>> tnt.mixin()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'TypingNamedTuple' object has no attribute 'mixin'

Hm, it looks like we can support this. I however don't think the proposed "patch" is the right way to fix it, since this makes the implementation with and without a mixin quite different.

Also I am not sure I will have time to work on this in the nearest month.

New changeset a94e6272f16381349dbed74cdb738ec8ae23b4fe by Serhiy Storchaka in branch 'master':
bpo-36517: Raise error on multiple inheritance with NamedTuple (GH-19363)
https://github.com/python/cpython/commit/a94e6272f16381349dbed74cdb738ec8ae23b4fe

The patch PR blocks out a useful idiom: generic Nametuple

>>> class LLNode(NamedTuple, Generic[T]):
...     value :T
...     next: Optional[LLNode[T]]

I put forward that, at the least, NamedTuple should accept do-nothing bases like Generic.