Mark ErrorCodes as serializable (#15218)

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

---------

Co-authored-by: Valentin Stanciu <250871+svalentin@users.noreply.github>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

Author: 4 people

mypy/errorcodes.py

@@ -8,10 +8,13 @@
 from collections import defaultdict
 from typing_extensions import Final
 
+from mypy_extensions import mypyc_attr
+
 error_codes: dict[str, ErrorCode] = {}
 sub_code_map: dict[str, set[str]] = defaultdict(set)
 
 
+@mypyc_attr(serializable=True)
 class ErrorCode:
     def __init__(
         self,