Add dataclass InitVar test case

[JukkaL]

This test case covers a recent change in functionality:
if an attribute is defined as an InitVar, it can't be
assigned to. Previously this was supported (probably
by accident).

[AlexWaygood]

This makes sense to me. (If you use slots=True on 3.10, does it generate a slot for InitVars, out of interest, do you know?)

[AlexWaygood]

If you use slots=True on 3.10, does it generate a slot for InitVars, out of interest, do you know?

I've confirmed locally that slots for InitVars are not autogenerated at runtime with slots=True, so that supports this behaviour even more:

>>> from dataclasses import dataclass, InitVar
>>> @dataclass(slots=True)
... class Foo:
...     bar: InitVar[str]
...     baz: InitVar[int] = 0
...
>>> f = Foo('hi', 5)
>>> f.__slots__
()
>>> f.bar = 'bye'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute 'bar'
>>> f.baz = 6
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object attribute 'baz' is read-only

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉