docs: document minimum python versions

[jbedard]

Changes are visible to end-users: no

Test plan

• Covered by existing test cases

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-16T23:36:45.763457Z	5a294f6	PR opened

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[jbedard]

It's possibly we'll want to change this to allow 3.10 but lets just stick with it and document it for now.

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📅 Wed Sep 16 23:35:29 UTC 2026

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 5a294f629c

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[chatgpt-codex-connector]

Avoid claiming all older-version targets fail at startup

With a fetched pre-3.10 interpreter and a compatible user entry point, generic py_binary and py_test targets do not load the 3.10-only test runners: py/private/py_venv/py_venv_exec.bzl constructs a native launcher that directly executes the venv interpreter with the user's main (lines 94-118). Such targets can therefore start successfully; the syntax restriction applies to specific maintained helpers such as the pytest/unittest runners and build tools. Describe older interpreters as unsupported, or scope the startup-failure statement to targets that actually execute those helpers.

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[github-actions]

py_binary startup benchmark

Version	Time (ms)	vs BCR	vs main	Build (s)
BCR 1.11.7	162.2/161.6 ±2.4	—	—	1.77
main	57.1/57.1 ±0.7	-64.7%	—	1.97
PR	57.2/57.1 ±1.0	-64.7%	+0.0%	1.66

Time = mean/median ±stddev.
Measured with hyperfine --warmup 5 --runs 50 --shell=none on Linux
Gate: PR vs HEAD main median (threshold: 10%, and must exceed the 2×SE noise floor, here 0.6%). BCR is shown only as a historical baseline.
Build time: cold bazel build //:bench with isolated output base, no disk cache; external repos prefetched so network is excluded.

sys.path quality

Version	entries	sp roots	dupes
BCR 1.11.7	6	1	0
main	7	2	0
PR	7	2	0

sys.path quality measured by bench_syspath inside the assembled venv: sys.path entries, distinct site-packages roots, duplicate realpaths. Duplicates indicate symlink redundancy; many roots suggest an inefficient venv layout.

Bazel analysis benchmark

Version	Time (ms)	vs BCR	vs main	Targets	Actions	Configured (ext)	Cfg/target	Actions/cfg
BCR 2.0.0-alpha.6	1213/1181 ±199	—	—	404	20613	7380 (6675)	18.3	2.79
main	1165/1133 ±210	-4.1%	—	404	18363	7379 (6674)	18.3	2.49
PR	1171/1174 ±154	-0.6%	+3.7%	404	18363	7379 (6674)	18.3	2.49

Measured with hyperfine --warmup 1 --runs 10 on Linux
Time = mean/median ±stddev. Cfg = configured targets; (ext) = the count in external repos (the @pypi hub machinery).
Gate: PR vs HEAD main median (threshold: 10%, and must exceed the 2×SE noise floor, here 14.5%). BCR is shown only as a historical baseline.
Command: warm-server bazel build --nobuild //workspace/..., analysis cache discarded each run via a fresh --action_env value; no disk cache.
Workload: 100 py_test targets — 2 test files/package in file generation mode (one venv per test file); a single dep_group.

py_image_layer benchmark

Scenario	Version	Time (s)	Actions	vs BCR	vs main
analysis	BCR 2.0.0-alpha.6	0.83/0.78 ±0.20	747	—	—
analysis	main	0.82/0.78 ±0.14	668	+0.8%	—
analysis	PR	0.79/0.75 ±0.13	668	-2.9%	-3.6%
1p source	BCR 2.0.0-alpha.6	0.81/0.80 ±0.03	4	—	—
1p source	main	0.83/0.82 ±0.05	5	+1.8%	—
1p source	PR	0.81/0.80 ±0.04	5	-0.0%	-1.8%
3p wheel	BCR 2.0.0-alpha.6	0.94/0.94 ±0.02	5	—	—
3p wheel	main	0.97/0.97 ±0.03	6	+3.9%	—
3p wheel	PR	0.97/0.97 ±0.02	6	+3.7%	-0.2%

Time = mean/median ±stddev.
Measured with hyperfine on Linux, building //workspace:image_layers (10 binaries, ~30-wheel dep pool, grouped first-party/pip/interpreter tier) with isolated output base, no disk cache.
Scenarios: analysis = warm-server bazel build --nobuild, re-analyzed each run via a fresh --action_env value; incrementals run against a built state with warm analysis: source = append to the last package's lib.py, wheel = rewrite click post_install_patches content.
Actions: for Analysis, the total action count behind the image target from aquery deps(...); for incrementals, actions re-executed for the mutation, from a single instrumented run's BEP build metrics (deterministic; per-mnemonic breakdown in the *-actions.json artifacts). Informational only, not gated.
Gate: PR vs HEAD main median per scenario (threshold: 10%, and must exceed the 2×SE noise floor). BCR is shown only as a historical baseline.