Mark ErrorCodes as serializable

[svalentin]

When starting mypy daemon, we pickle options. Error codes are part of options and need to be pickle-able. This decorator is needed for this to be possible with mypyc.

Fixes #14671

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Thanks!

[cdce8p]

This PR seems to cause a new crash with custom plugins - only in compiled mode. I noticed it with pydantic. (Tested with Python 3.11)

pydantic==1.10.7

# config.ini
plugins = pydantic.mypy

# t.py
from pydantic import BaseModel

reveal_type(1)

mypy --config-file=config.ini --show-traceback t.py

Traceback

/.../pydantic/decorator.py:226: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.4.0+dev.c1fb57d38116adf6c1c1ea5c4b998cae597ec156
Traceback (most recent call last):
  File "mypy/semanal.py", line 6281, in accept
  File "mypy/nodes.py", line 1139, in accept
  File "mypy/semanal.py", line 1574, in visit_class_def
  File "mypy/semanal.py", line 1654, in analyze_class
  File "mypy/semanal.py", line 1681, in analyze_class_body_common
  File "mypy/semanal.py", line 1764, in apply_class_plugin_hooks
  File "/.../pydantic/mypy.py", line 154, in _pydantic_model_class_maker_callback
    transformer.transform()
  File "/.../pydantic/mypy.py", line 313, in transform
    config = self.collect_config()
             ^^^^^^^^^^^^^^^^^^^^^
  File "/.../pydantic/mypy.py", line 360, in collect_config
    config.update(self.get_config_update(substmt))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/.../pydantic/mypy.py", line 568, in get_config_update
    error_invalid_config_value(lhs.name, self._ctx.api, substmt)
  File "/.../pydantic/mypy.py", line 781, in error_invalid_config_value
    api.fail(f'Invalid value for "Config.{name}"', context, code=ERROR_CONFIG)
  File "mypy/semanal.py", line 6262, in fail
  File "mypy/errors.py", line 436, in report
  File "mypy/errors.py", line 478, in add_error_info
  File "mypy/errors.py", line 577, in is_ignored_error
  File "mypy/errors.py", line 604, in is_error_code_enabled
AttributeError: attribute 'sub_code_of' of 'ErrorCode' undefined
/.../pydantic/decorator.py:226: : note: use --pdb to drop into pdb

From what I can tell, the pydantic plugin always wants to emit an error (which isn't actually shown - this is probably an issue with the plugin itself) but when it calls .fail(...) with a custom ErrorCode mypy crashes. This is the Error Code

ERROR_CONFIG = ErrorCode('pydantic-config', 'Invalid config value', 'Pydantic')