[ty] Detect illegal multiple inheritance with NamedTuple

[AlexWaygood]

Summary

This fails at runtime:

from typing import NamedTuple

class Foo(NamedTuple, object): ...

and the typing spec mandates that we should detect that, but we currently don't. This PR adds a new lint for detecting this mistake.

Test Plan

Mdtest/snapshots

[github-actions]

Diagnostic diff on typing conformance tests

Changes were detected when running ty on typing conformance tests

--- old-output.txt	2025-08-18 12:00:59.100097702 +0000
+++ new-output.txt	2025-08-18 12:01:01.560109174 +0000
@@ -672,6 +672,7 @@
 namedtuples_define_class.py:95:1: error[type-assertion-failure] Argument does not have asserted type `int | float`
 namedtuples_define_class.py:96:1: error[type-assertion-failure] Argument does not have asserted type `int | float`
 namedtuples_define_class.py:98:19: error[invalid-argument-type] Argument is incorrect: Expected `str`, found `float`
+namedtuples_define_class.py:105:24: error[invalid-named-tuple] NamedTuple class `Unit` cannot use multiple inheritance except with `Generic[]`
 namedtuples_type_compat.py:22:1: error[invalid-assignment] Object of type `Point` is not assignable to `tuple[int, int]`
 namedtuples_type_compat.py:23:1: error[invalid-assignment] Object of type `Point` is not assignable to `tuple[int, str, str]`
 namedtuples_usage.py:34:7: error[index-out-of-bounds] Index 3 is out of bounds for tuple `Point` with length 3
@@ -860,5 +861,5 @@
 typeddicts_operations.py:60:1: error[type-assertion-failure] Argument does not have asserted type `str | None`
 typeddicts_type_consistency.py:101:1: error[invalid-assignment] Object of type `Unknown | None` is not assignable to `str`
 typeddicts_usage.py:40:24: error[invalid-type-form] The special form `typing.TypedDict` is not allowed in type expressions. Did you mean to use a concrete TypedDict or `collections.abc.Mapping[str, object]` instead?
-Found 861 diagnostics
+Found 862 diagnostics
 WARN A fatal error occurred while checking some files. Not all project files were analyzed. See the diagnostics list above for details.

[github-actions]

mypy_primer results

No ecosystem changes detected ✅
No memory usage changes detected ✅

[AlexWaygood]

(The new typing conformance suite diagnostic is correct!)

[sharkdp]

Nice — thank you.