pytest with importlib & dataclass & __future__ annotations crash

[yoav-orca]

I'm transitioning from pytest regular imports to --import-mode=importlib imports due to namespace collisions in tests. And I'm getting the following crash, no plugins installed, pytest 6.1.1 CPython 3.8.6, the issue reproduces on Mac & Linux.

File names test_xx.py:

from __future__ import annotations

from dataclasses import dataclass

@dataclass
class TestHelper:
    value: str

running pytest --import-mode=importlib test_xx.py and I get the following crash:

==================================================== test session starts ====================================================
platform darwin -- Python 3.8.5, pytest-6.1.1, py-1.9.0, pluggy-0.13.1
rootdir: /private/tmp/qwer/dir
collected 0 items / 1 error

========================================================== ERRORS ===========================================================
________________________________________________ ERROR collecting test_xx.py ________________________________________________
test_xx.py:6: in <module>
    class TestHelper:
/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/dataclasses.py:1019: in dataclass
    return wrap(cls)
/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/dataclasses.py:1011: in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/dataclasses.py:861: in _process_class
    cls_fields = [_get_field(cls, name, type)
/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/dataclasses.py:861: in <listcomp>
    cls_fields = [_get_field(cls, name, type)
/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/dataclasses.py:712: in _get_field
    and _is_type(f.type, cls, typing, typing.ClassVar,
/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/dataclasses.py:658: in _is_type
    ns = sys.modules.get(cls.__module__).__dict__
E   AttributeError: 'NoneType' object has no attribute '__dict__'
================================================== short test summary info ==================================================
ERROR test_xx.py - AttributeError: 'NoneType' object has no attribute '__dict__'
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
===================================================== 1 error in 0.28s ======================================================

pip list:

Package    Version
---------- -------
attrs      20.2.0
iniconfig  1.0.1
packaging  20.4
pip        20.1.1
pluggy     0.13.1
py         1.9.0
pyparsing  2.4.7
pytest     6.1.1
setuptools 50.3.0
six        1.15.0
toml       0.10.1
wheel      0.35.1