Issue 37868: `is_dataclass` returns `True` if `getattr` always succeeds.

Given a class `A` that overloads `__getattr__`

```
class A:
  def __getattr__(self, key):
    return 0
```

An instance of this class is always identified as a dataclass.

```
from dataclasses import is_dataclass

a = A()
print(is_dataclass(a))
```

gives the output `True`.

Possible fix: check for the instance type.

```
is_dataclass(type(a))
```

does give the correct answer.

I'm guessing I'm looking up the attribute on the instance, not the class.

I am not sure that it is good idea to accept a type and an instance, but if it is a goal, is_dataclass() should be defined as:

def is_dataclass(obj):
    cls = obj if isinstance(obj, type) else type(obj)
    return hasattr(cls, _FIELDS)

_is_dataclass_instance() should be changed too:

def _is_dataclass_instance(obj):
    return hasattr(type(obj), _FIELDS)

Yeah, I agree it's not an awesome design to work with classes or instances, but it's documented that way. As soon as I write some tests I'll check this in.

New changeset b0f4dab8735f692bcfedcf0fa9a25e238a554bab by Eric V. Smith in branch 'master':
bpo-37868: Improve is_dataclass for instances. (GH-15325)
https://github.com/python/cpython/commit/b0f4dab8735f692bcfedcf0fa9a25e238a554bab

New changeset 1271ee8187df31debda7c556882a51ec356ca534 by Miss Islington (bot) in branch '3.8':
bpo-37868: Improve is_dataclass for instances. (GH-15325)
https://github.com/python/cpython/commit/1271ee8187df31debda7c556882a51ec356ca534

New changeset 02c1457a036c2af3e91beb952afdb66d9c806435 by Miss Islington (bot) in branch '3.7':
bpo-37868: Improve is_dataclass for instances. (GH-15325)
https://github.com/python/cpython/commit/02c1457a036c2af3e91beb952afdb66d9c806435

nosy: + eric.smith