Replace individual workflows with one "checks" workflow (#4)

This uses re-actors/alls-green to make sure they all are passing, which makes
it simpler to configure the rules in GitHub for when a PR is allowed to be
merged.

.github/workflows/checks.yml

@@ -0,0 +1,91 @@
+---
+name: Checks
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  mypy:
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout the code
+        uses: actions/checkout@v4
+
+      - name: Install Poetry
+        run: pipx install poetry
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          cache: poetry
+
+      - name: Install dependencies
+        run: poetry install
+
+      - name: Run mypy
+        run: poetry run mypy .
+
+  pre-commit:
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+
+      - name: Pre-commit
+        uses: pre-commit/[email protected]
+
+  pytest:
+    runs-on: ubuntu-24.04
+
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version:
+          - "3.9"
+          - "3.10"
+          - "3.11"
+          - "3.12"
+          - "3.13-dev"
+
+    steps:
+      - name: Checkout the code
+        uses: actions/checkout@v4
+
+      - name: Install Poetry
+        run: pipx install poetry
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: poetry
+
+      - name: Install dependencies
+        run: poetry install
+
+      - name: Run pytest
+        run: poetry run pytest
+
+  all-checks-passed:
+    if: always()
+
+    needs:
+    - mypy
+    - pre-commit
+    - pytest
+
+    runs-on: ubuntu-24.04
+
+    steps:
+    - name: Decide whether the needed jobs succeeded or failed
+      uses: re-actors/alls-green@release/v1
+      with:
+        jobs: ${{ toJSON(needs) }}

.python-version

@@ -0,0 +1 @@
+3.12