test: add lint & test tooling for Aider

Aider (the AI coding assistant) can run and react to linter and test
output automatically. I use Aider occasionally, so this helps in that
workflow.

.env

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+AIDER_LINT_CMD=./run-lint.sh
+AIDER_TEST_CMD=./run-tests.sh

run-lint.sh

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+#!/usr/bin/env bash
+
+VENV=$HOME/.virtualenvs/darker
+PIP=${VENV}/bin/pip
+[ ! -f ${PIP} ] && python -m venv ${VENV} && ${PIP} install -U pip
+${PIP} install -q -e '.[color,test,release]'
+${PIP} uninstall -q -y ruff
+
+ensure() { command -v $1 >/dev/null || ${PIP} install -q $1; }
+
+ensure black
+ensure codespell
+
+errors=0
+
+source ${VENV}/bin/activate
+for file in "$@"; do
+    case "$file" in
+        *.py)
+            darker --quiet --isort --revision origin/main "$file" || errors=$?
+            graylint --quiet --revision origin/main \
+              --lint mypy \
+              --lint "ruff check" \
+              --lint "codespell" \
+              "$file" || errors=$?
+            ;;
+        *.sh|*.md|*.rst|*.txt)
+            codespell "$file" || errors=$?
+            ;;
+    esac
+done
+deactivate
+
+exit $errors

run-tests.sh

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+#!/usr/bin/env bash
+
+VENV=$HOME/.virtualenvs/darker
+PIP=${VENV}/bin/pip
+[ ! -f ${PIP} ] && python -m venv ${VENV} && ${PIP} install -U pip
+${PIP} install -q -e '.[color,test,release]'
+
+errors=0
+${VENV}/bin/pytest || errors=$?
+
+exit $errors