chore(specs): avoid using an itertools object in the BaseTest pydantic model (#1414)

* chore(specs): avoid using an itertools object as a pydantic model attribute

Pydantic does a deepcopy of the model upon validation which triggers the following deprecation warning:
```
tests/istanbul/eip1344_chainid/test_chainid.py: 24 warnings
  /usr/lib/python3.12/copy.py:151: DeprecationWarning: Pickle, copy, and deepcopy support will be removed from itertools in Python 3.14.
    rv = reductor(4)
```

* docs: update changelog

---------

Co-authored-by: Mario Vega <[email protected]>

Author: danceratopz

docs/CHANGELOG.md

@@ -10,6 +10,9 @@ Test fixtures for use by clients are available for each release on the [Github r
 
 ### 🛠️ Framework
 
+#### `fill`
+
+- 🐞 Fix `DeprecationWarning: Pickle, copy, and deepcopy support will be removed from itertools in Python 3.14.` by avoiding use `itertools` object in the spec `BaseTest` pydantic model ([#1414](https://github.com/ethereum/execution-spec-tests/pull/1414)).
 - ✨ The `static_filler` plug-in now has support for static state tests (from [GeneralStateTests](https://github.com/ethereum/tests/tree/develop/src/GeneralStateTestsFiller)) ([#1362](https://github.com/ethereum/execution-spec-tests/pull/1362)).
 
 ### 📋 Misc

src/ethereum_test_specs/base.py

@@ -2,10 +2,9 @@
 
 from abc import abstractmethod
 from functools import reduce
-from itertools import count
 from os import path
 from pathlib import Path
-from typing import Callable, ClassVar, Dict, Generator, Iterator, List, Optional, Sequence
+from typing import Callable, ClassVar, Dict, Generator, List, Optional, Sequence
 
 import pytest
 from pydantic import BaseModel, Field
@@ -49,7 +48,7 @@ class BaseTest(BaseModel):
 
     # Transition tool specific fields
     t8n_dump_dir: Path | None = Field(None, exclude=True)
-    _t8n_call_counter: Iterator[int] = count(0)
+    t8n_call_counter: int = Field(0, exclude=True)
 
     supported_fixture_formats: ClassVar[Sequence[FixtureFormat | LabeledFixtureFormat]] = []
     supported_execute_formats: ClassVar[Sequence[LabeledExecuteFormat]] = []
@@ -113,9 +112,11 @@ def get_next_transition_tool_output_path(self) -> str:
         """Return path to the next transition tool output file."""
         if not self.t8n_dump_dir:
             return ""
+        current_value = self.t8n_call_counter
+        self.t8n_call_counter += 1
         return path.join(
             self.t8n_dump_dir,
-            str(next(self._t8n_call_counter)),
+            str(current_value),
         )