Remove iso8601 dependency

[waketzheng]

Description

This removes the mandatory runtime dependency on iso8601 and keeps datetime parsing centralized through
tortoise.timezone.parse_datetime.

The new behavior is:

• Prefer ciso8601.parse_datetime when available.
• Fall back to tortoise._iso8601.parse_datetime when ciso8601 cannot be imported.
• Keep the fallback parser outside timezone.py, so timezone handling stays small and focused.
• Reuse the shared parser from field conversion and Oracle timestamp conversion.
• Keep support for common ISO 8601 forms accepted by ciso8601, including Z/z UTC suffixes, compact
  datetime strings, week dates, and ordinal dates such as 2020-230 and 2020230.

This also updates the lockfile, removes direct test dependency on iso8601, and adds Windows CI coverage
for the accel extra now that ciso8601 is no longer excluded on Windows CPython.

Motivation and Context

iso8601 was only used as a fallback parser when ciso8601 was unavailable. That makes it a runtime dependency for a narrow path that can now be covered by Python's standard library on Tortoise ORM's supported Python versions.

Keeping iso8601 has become hard to justify:

• It adds an extra runtime dependency for all users, even though datetime parsing is the only reason it is needed.
• Its maintenance cadence is slow, which increases long-term dependency risk for a core parsing path.
• It duplicates functionality that is now largely available through datetime.fromisoformat().
• It adds supply-chain and lockfile surface area for behavior we can implement locally in a small, explicit compatibility wrapper.
• It creates a split fallback story: accelerated installs use ciso8601, while non-accelerated installs depend on a separate third-party parser with subtly different behavior.

This change keeps the fast path unchanged for users who install accel, while making the non-accelerated path self-contained and easier to audit. The fallback parser intentionally preserves the common ciso8601 formats that matter for existing Tortoise ORM behavior, so removing iso8601 should not narrow normal datetime input support.

The ciso8601 platform marker was also relaxed to allow Windows CPython installs. A Windows CI smoke job was added to verify that the accel extra installs and that the accelerated parser is actually selected on Windows.

How Has This Been Tested?

Tested locally on macOS with Python 3.14.2.

GitHub Actions for the latest pushed commit are passing:

• ci: passed
• CodSpeed: passed

Additional coverage added:

• Verifies fallback parser behavior directly.
• Verifies fallback import behavior when ciso8601 is unavailable.
• Verifies fallback parser matches ciso8601.parse_datetime for common supported datetime formats.
• Adds Windows CI smoke coverage for ciso8601 via the accel extra.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing waketzheng:remove-iso8601 (3e3806b) with develop (ac9ef69)}