github actions: Try to parameterise testing

Having separate action files for different Python versions sucks.

.github/workflows/main.yml

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+name: Pre-merge and post-merge tests
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+    inputs:
+      run-slow:
+        description: "Whether to run slow tests (normally they are skipped)"
+        type: boolean
+        required: false
+        default: false
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Run tests
+    uses: ./.github/workflows/test.yml
+    # https://wildwolf.name/github-actions-how-to-avoid-running-the-same-workflow-multiple-times/
+    if: >
+      github.event_name != 'pull_request'
+      || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
+    strategy:
+      matrix:
+        python:
+        - "3.6"
+        - "3.8"
+        - "3.12"
+        platform:
+        # This package is supposed to be OS-independent and is unlikely to have
+        # OS-specific bugs, so we conserve runner usage by only testing on Linux
+        # during pre-merge and post-merge testing. If it works on Linux, it'll
+        # probably work on Mac and Windows too. But if an OS-specific bug does
+        # slip through, we should catch it in pre-release testing.
+        - ubuntu-latest
+        force-minimum-dependencies:
+        - false
+        exclude:
+        # Python 3.5 does not run on ubuntu-latest
+        - python: "3.5"
+          platform: ubuntu-latest
+        include:
+        - python: "3.5"
+          platform: ubuntu-20.04
+          force-minimum-dependencies: false
+        # For testing forced minimum deps, use the latest stable version of Python
+        # on which those dependencies can be installed
+        - python: "3.12"
+          platform: ubuntu-latest
+          force-minimum-dependencies: true
+    with:
+      python-version: ${{ matrix.python }}
+      platform: ${{ matrix.platform }}
+      force-minimum-dependencies: ${{ matrix.force-minimum-dependencies }}
+      run-slow: ${{ inputs.run-slow || false }}
+
+  check:  # This job does nothing and is only used for the branch protection
+    # https://wildwolf.name/github-actions-how-to-avoid-running-the-same-workflow-multiple-times/
+    if: >
+      github.event_name != 'pull_request'
+      || github.event.pull_request.head.repo.full_name != github.event.pull_request.base.repo.full_name
+
+    needs:
+    - test
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Decide whether the needed jobs succeeded or failed
+      uses: re-actors/alls-green@release/v1
+      with:
+        jobs: ${{ toJSON(needs) }}

.github/workflows/test.yml

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+name: Reusable workflow that runs all tests
+
+on:
+  workflow_call:
+    inputs:
+      python-version:
+        type: string
+        required: true
+      platform:
+        type: string
+        required: true
+      # Set this flag to test that the package works with the listed minimum
+      # versions of dependencies.
+      force-minimum-dependencies:
+        type: boolean
+        required: false
+        default: false
+      # Set this flag to run even the slow tests
+      run-slow:
+        type: boolean
+        required: false
+        default: true
+
+permissions:
+  contents: read
+
+env:
+  # Environment variables to support color support (jaraco/skeleton#66):
+  # Request colored output from CLI tools supporting it. Different tools
+  # interpret the value differently. For some, just being set is sufficient.
+  # For others, it must be a non-zero integer. For yet others, being set
+  # to a non-empty value is sufficient. For tox, it must be one of
+  # <blank>, 0, 1, false, no, off, on, true, yes. The only enabling value
+  # in common is "1".
+  FORCE_COLOR: 1
+  # Recognized by the `py` package, dependency of `pytest` (must be "1")
+  PY_COLORS: 1
+
+  # Suppress noisy pip warnings
+  PIP_DISABLE_PIP_VERSION_CHECK: 'true'
+  PIP_NO_PYTHON_VERSION_WARNING: 'true'
+  PIP_NO_WARN_SCRIPT_LOCATION: 'true'
+
+
+jobs:
+  test:
+    runs-on: ${{ inputs.platform }}
+    steps:
+      - uses: actions/checkout@v3
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ inputs.python-version }}
+          allow-prereleases: true
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install flake8 pytest coverage coverage-lcov toml pint
+        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+    - name: Lint with flake8
+      run: |
+        # stop the build if there are Python syntax errors or undefined names
+        flake8 .  --count --select=E9,F63,F7,F82 --show-source --statistics
+        # exit-zero treats all errors as warnings.
+        flake8 .  --count --exit-zero --max-complexity=10 --statistics
+    - name: Test with py.test
+      run: |
+        coverage run -m pytest
+        coverage-lcov
+    - name: Coveralls
+      uses: coverallsapp/github-action@master
+      with:
+        github-token: ${{ secrets.GITHUB_TOKEN }}
+        path-to-lcov: ./lcov.info