Make DateTime.astimezone timezone optional

[xiehuanyi]

Summary

Parsed DateTime objects reject value.astimezone() because the wrapper requires tz, while native datetime.astimezone() defaults to the local timezone. Give the wrapper the same optional argument, preserving its existing explicit-timezone behavior and wrapped result type.

Validation: the new regression fails on the unchanged source; all 1185 tests, including the pinned toml-test fixtures, pass after the change. Applicable pre-commit hooks pass. Mypy reports the identical six existing errors on the base and this branch (submodule generator excluded), with no new errors.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: GPT-6.1 Sol, xhigh
• Notes: The description and changes in tomlkit/items.py, tests/test_items.py and CHANGELOG.md were prepared with this agent.

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[Wkhanex]

I checked the changed astimezone method against stdlib behavior for naive, UTC-aware and ZoneInfo-aware datetimes. In all three controls, omitting the argument matched explicit None and the stdlib local-timezone conversion. The existing _new reconstruction path is retained by the patch. A naive input regression would complement the new UTC-aware test, since datetime.astimezone also permits naive inputs. I ran these checks on Python 3.12 using the exact method in a datetime subclass adapter; TOML parsing/trivia reconstruction and the full package tests were not run.