pyproject(mypy) Add mypy override for frozen_dataclass method-assign

why: Fix type checking errors in the custom frozen_dataclass implementation

what:
- Added targeted mypy configuration override to disable method-assign errors
- Only scoped to libtmux._internal.frozen_dataclass module
- Preserves strict type checking across the rest of the codebase

refs: Enables inheritance from mutable to immutable dataclasses

Author: tony

pyproject.toml

@@ -128,6 +128,10 @@ files = [
   "tests",
 ]
 
+[[tool.mypy.overrides]]
+module = "libtmux._internal.frozen_dataclass"
+disable_error_code = ["method-assign"]
+
 [tool.coverage.run]
 branch = true
 parallel = true