New python312.yml, based on upstream.yml, but with many tweaks.

.github/workflows/python312.yml

@@ -0,0 +1,125 @@
+name: Python312
+
+on:
+  workflow_dispatch:
+  pull_request:
+
+concurrency:
+  group: python312-${{ github.ref }}
+  cancel-in-progress: false
+
+env:
+  PIP_ONLY_BINARY: numpy
+  PYTEST_TIMEOUT: 300
+
+jobs:
+  standard:
+    name: "🐍 3.12 latest • ubuntu-latest • x64"
+    runs-on: ubuntu-latest
+    # if: "contains(github.event.pull_request.labels.*.name, 'python dev')"
+
+    steps:
+    - name: Show env
+      run: env
+
+    - uses: actions/checkout@v3
+
+    - name: Setup Python 3.12
+      uses: actions/setup-python@v4
+      with:
+        python-version: "3.12-dev"
+
+    - name: Setup Boost
+      run: sudo apt-get install libboost-dev
+
+    - name: Update CMake
+      uses: jwlawson/actions-setup-cmake@v1.13
+
+    - name: Run pip installs
+      run: |
+        python -m pip install --upgrade pip
+        python -m pip install -r tests/requirements.txt
+
+    - name: Show CMake version
+      run: cmake --version
+
+    # FIRST BUILD
+    - name: Configure C++11
+      run: >
+        cmake -S . -B .
+        cmake -S . -B build11
+        -DCMAKE_VERBOSE_MAKEFILE=ON
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=11
+
+    - name: Build C++11
+      run: cmake --build build11 -j 2
+
+    - name: Python tests C++11
+      run: cmake --build build11 --target pytest -j 2
+
+    - name: C++ tests C++11
+      run: cmake --build build11 --target cpptest -j 2
+
+    - name: Interface test C++11
+      run: cmake --build build11 --target test_cmake_build
+
+    - name: Clean directory
+      run: git clean -fdx
+
+    # SECOND BUILD
+    - name: Configure C++17
+      run: >
+        cmake -S . -B build17
+        -DCMAKE_VERBOSE_MAKEFILE=ON
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+
+    - name: Build C++17
+      run: cmake --build build17 -j 2
+
+    - name: Python tests C++17
+      run: cmake --build build17 --target pytest
+
+    - name: C++ tests C++17
+      run: cmake --build build17 --target cpptest
+
+    - name: Interface test C++17
+      run: cmake --build build17 --target test_cmake_build
+
+    - name: Clean directory
+      run: git clean -fdx
+
+    # THIRD BUILD
+    - name: Configure C++17 max DPYBIND11_INTERNALS_VERSION
+      run: >
+        cmake -S . -B build17max
+        -DCMAKE_VERBOSE_MAKEFILE=ON
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+        -DPYBIND11_INTERNALS_VERSION=10000000
+
+    - name: Build C++17 max DPYBIND11_INTERNALS_VERSION
+      run: cmake --build build17max -j 2
+
+    - name: Python tests C++17 max DPYBIND11_INTERNALS_VERSION
+      run: cmake --build build17max --target pytest
+
+    - name: C++ tests C++17 max DPYBIND11_INTERNALS_VERSION
+      run: cmake --build build17max --target cpptest
+
+    - name: Interface test
+      run: cmake --build build17max --target test_cmake_build
+
+    # Ensure the setup_helpers module can build packages using setuptools
+    - name: Setuptools helpers test
+      run: pytest tests/extra_setuptools
+
+    - name: Clean directory
+      run: git clean -fdx