feat: drop CMake 3.6 and below, modernize CMake

fix: include PYTHON_IS_DEBUG

.github/workflows/configure.yml

@@ -34,24 +34,10 @@ jobs:
 
     - name: Make build directories
       run: |
-        mkdir build2.8
         mkdir build3.7
+        mkdir build3.11
         mkdir build3.18
 
-    - name: Setup CMake 2.8
-      uses: jwlawson/actions-setup-cmake@v1.3
-      with:
-        cmake-version: 2.8
-
-    - name: Configure 2.8
-      working-directory: build2.8
-      run: >
-        cmake --version &&
-        cmake ..
-        -DPYBIND11_WERROR=ON
-        -DDOWNLOAD_CATCH=ON
-        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
-
     - name: Setup CMake 3.7
       uses: jwlawson/actions-setup-cmake@v1.3
       with:
@@ -60,7 +46,19 @@ jobs:
     - name: Configure 3.7
       working-directory: build3.7
       run: >
-        cmake --version &&
+        cmake ..
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DPYTHON_EXECUTABLE=$(python -c "import sys; print(sys.executable)")
+
+    - name: Setup CMake 3.11
+      uses: jwlawson/actions-setup-cmake@v1.3
+      with:
+        cmake-version: 3.11
+
+    - name: Configure 3.11
+      working-directory: build3.11
+      run: >
         cmake ..
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
@@ -74,7 +72,6 @@ jobs:
     - name: Configure 3.18
       working-directory: build3.18
       run: >
-        cmake --version &&
         cmake ..
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON