build: run flake8 and pip under the same interpreter as the tests

[sibson]

What & why

make lint ran the bare flake8 console script, which resolves through PATH
and need not be the interpreter make unittests and make setup use. In this
repo's web-session container it resolved to a uv tool venv
(/root/.local/share/uv/tools/flake8/) that had no plugins, while
flake8-black and flake8-isort were installed into a different Python
entirely.

The failure is silent in the worst way: flake8 runs, finds no plugins, checks
only pycodestyle/pyflakes, and exits 0. Both black and isort — the checks CI
actually enforces — are skipped, and setup.cfg's black-config = ./pyproject.toml is ignored because no plugin registers that option. make lint passes locally and the lint job fails on the PR.

Fix

Invoke through python -m so the linter binds to the same interpreter as
everything else:

lint:
	python -m flake8 redbeat tests

setup had the same split — line 1 used python -m pip, the next two used a
bare pip — so those are now python -m pip too. That inconsistency is
plausibly how the two-interpreter state arose in the first place.

Test plan

• make lint now reports flake8 7.3.0 (flake8-black: 0.4.0, flake8-isort: 7.0.0, …) and exits 0 with the plugins genuinely loaded — verified against a
  scratch file that BLK100, I001 and I003 all fire, and that
  skip-string-normalization from pyproject.toml is honored rather than black
  rewriting single quotes.
• Zero violations on current main; nothing reformatted.
• make test passes (89 tests).

No CHANGES.txt entry — this is build tooling, not a user-visible change.

Note on CI

CI is unaffected: .github/workflows/ci.yml installs flake8 flake8-black flake8-isort into the job's only Python and calls flake8 directly, so it was
always running the real checks. This only fixes local/web-session runs, where
the mismatch made make lint a no-op.

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