Python 3.14.0a3: TypeError: Extra positional arguments provided

[hroncok]

Description

Hello. In Fedora, we try to build this package with Python 3.14.0a3.

We see test errors, on 0.19.0 as well as the current main branch. To reproduce, I did:

[msgspec (main)]$ uv venv --python=python3.14 venv3.14
(venv3.14) [msgspec (main)]$ uv pip install .[test]
Using Python 3.14.0a3 environment at: venv3.14
Resolved 9 packages in 1.12s
   Built msgspec @ file:///home/.../msgspec
   Built msgpack==1.1.0
   Built pyyaml==6.0.2
Prepared 5 packages in 5.98s
Installed 9 packages in 8ms
 + attrs==24.3.0
 + iniconfig==2.0.0
 + msgpack==1.1.0
 + msgspec==0.19.0 (from file:///home/.../msgspec)
 + packaging==24.2
 + pluggy==1.5.0
 + pytest==8.3.4
 + pyyaml==6.0.2
 + tomli-w==1.1.0
(venv3.14) [msgspec (main)]$ pytest
============================= test session starts ==============================
platform linux -- Python 3.14.0a3, pytest-8.3.4, pluggy-1.5.0
rootdir: /home/.../msgspec
configfile: setup.cfg
collected 5762 items / 3 errors / 2 skipped

==================================== ERRORS ====================================
____________________ ERROR collecting tests/test_struct.py _____________________
tests/test_struct.py:842: in <module>
    FrozenPoint(1, 2),
E   TypeError: Extra positional arguments provided
_____________________ ERROR collecting tests/test_toml.py ______________________
tests/test_toml.py:125: in <module>
    (ExStruct(1, "two"), ExStruct),
E   TypeError: Extra positional arguments provided
_____________________ ERROR collecting tests/test_yaml.py ______________________
tests/test_yaml.py:105: in <module>
    (ExStruct(1, "two"), ExStruct),
E   TypeError: Extra positional arguments provided
=========================== short test summary info ============================
ERROR tests/test_struct.py - TypeError: Extra positional arguments provided
ERROR tests/test_toml.py - TypeError: Extra positional arguments provided
ERROR tests/test_yaml.py - TypeError: Extra positional arguments provided
!!!!!!!!!!!!!!!!!!! Interrupted: 3 errors during collection !!!!!!!!!!!!!!!!!!!!
========================= 2 skipped, 3 errors in 0.40s =========================