CI: Fail on any warnings with gcc 13

Author: rhaschke

.github/workflows/ci.yml

@@ -493,9 +493,9 @@ jobs:
           - { gcc: 9, std: 20 }
           - { gcc: 10, std: 17 }
           - { gcc: 10, std: 20 }
-          - { gcc: 13, std: 20 }
+          - { gcc: 13, std: 20, cxx_flags: "-Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls" }
 
-    name: "🐍 3 • GCC ${{ matrix.gcc }} • C++${{ matrix.std }}• x64"
+    name: "🐍 3 • GCC ${{ matrix.gcc }} • C++${{ matrix.std }} • x64${{ matrix.cxx_flags && ' • cxx_flags' || '' }}"
     container: "gcc:${{ matrix.gcc }}"
 
     steps:
@@ -517,6 +517,7 @@ jobs:
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DCMAKE_CXX_STANDARD=${{ matrix.std }}
+        -DCMAKE_CXX_FLAGS="${{ matrix.cxx_flags }}"
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 
     - name: Build