fix: pin the Black target version to the minimum supported Python

[razor-x]

Problem

Every Lint job on beta emits a warning, on all four matrix legs (3.11–3.14):

Warning: Python 3.14 cannot parse code formatted for Python 3.15. To fix this: run Black
with Python 3.15, set --target-version to py314, or use --fast to skip the safety check.
Black's safety check verifies equivalence by parsing the AST, which fails when the running
Python is older than the target version.

Latest beta run: 31718873837. It also reproduces locally via just lint.

Cause

There was no [tool.black] section, so Black inferred its target versions from project.requires-python. Because that bound is open-ended (>=3.11), it expanded to every version Black knows about:

$ black --check --verbose seam/__init__.py
target_version: ['py311', 'py312', 'py313', 'py314', 'py315']

Black's safety check re-parses its own output with the stdlib ast module, using the highest target version. No interpreter from 3.11 through 3.14 can produce a 3.15 AST, so the check is skipped and the warning is printed.

Impact

Formatting was never wrong, and lint was never failing. Black only emits syntax common to all of its targets, so it was already formatting for the 3.11 floor. Verified two ways:

• black --diff --target-version py315 . and --target-version py311 . produce byte-identical output across the repo.
• Black still reformats correctly under the old config (checked against a deliberately unformatted file).

What was actually lost is the AST equivalence check — Black's guard against its own output changing program semantics. That guard was silently off for every lint run.

Change

[tool.black]
target-version = ["py311"]

Pinning the floor explicitly restores the safety check and silences the warning, with no formatting change.

Verification

$ black --check --verbose seam/__init__.py
target_version: ['py311']          # no warning

$ black --check .
All done! ✨ 🍰 ✨
110 files would be left unchanged.  # exit 0

Note for reviewers

This value needs bumping alongside any future increase to requires-python. The alternative — leaving the warning in place — keeps one less thing to maintain but leaves Black's safety check disabled, so pinning seemed the better trade.

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Generated by Claude Code