Update typeddicts_class_syntax.py test to use a custom metaclass.

[rchen152]

The TypedDict chapter of the spec says (emphasis mine):

It is invalid to specify a base class other than TypedDict, Generic, or another TypedDict type in a class-based TypedDict definition. It is also invalid to specify a custom metaclass.

It looks like most type checkers interpret this to mean that the appearance of metaclass=... is illegal in a TypedDict definition.

Pyrefly interprets this to mean that a non-type metaclass is illegal, so a redundant metaclass=type is fine.

IMO either interpretation is defensible, but the conformance test currently uses metaclass=type, which only accommodates the first. This PR changes the test to use a non-type metaclass, which is unambiguously an error.

This doesn't actually change pyrefly's conformance results because pyrefly has a bug that causes it to emit a different, nonsensical error on the metaclass=type line, which was mistakenly taken as pyrefly passing the test. I'm in the process of fixing that bug, which is how I discovered that pyrefly behaves slightly differently from the other checkers here.

[davidhalter]

This seems to map with runtime behavior:

>>> from typing import *
>>> class BadTypedDict2(TypedDict, metaclass=type): pass
... 
>>> class CustomMeta(type):
...     pass
... 
>>> class BadTypedDict2(TypedDict, metaclass=CustomMeta): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases