fix: reorder None to the end of type unions (RUF036)

[syzayd]

What and why

ruff isn't version-pinned in pyproject.toml, and a newer release started enforcing RUF036 ("None not at the end of a type union"). That currently flags 14 pre-existing annotations across tenacity/__init__.py, tenacity/asyncio/__init__.py, and tenacity/retry.py, and breaks the lint CI job for any PR against main right now (verified: the last green run on main, 2026-08-01, predates this).

Split out on its own here (was flagged as noise when bundled into #668) so it can be reviewed independently.

Change

Pure reordering, no behavior change: uv run ruff check --fix --unsafe-fixes . then ruff format ..

Verification

$ ruff check .
All checks passed!

$ ruff format --check .
20 files already formatted

$ mypy tenacity tests
Success: no issues found in 19 source files

$ pytest -q
173 passed, 1 warning, 12 subtests passed