ci: test C++17 on MSVC 2017

Author: henryiii

.github/workflows/ci.yml

@@ -30,15 +30,11 @@ jobs:
             arch: x64
             max-cxx-std: 17
             args: "-DPYBIND11_FINDPYTHON=ON"
-          - runs-on: macos-latest
-            python: 3.7
-            arch: x64
-            max-cxx-std: 17
-            args: "-DPYBIND11_FINDPYTHON=ON"
           - runs-on: windows-2016
             python: 3.7
             arch: x86
-            max-cxx-std: 14
+            max-cxx-std: 17
+            args: "-DCMAKE_CXX_FLAGS=//permissive- //DWIN32 //D_WINDOWS //W3 //GR //EHsc"
           - runs-on: windows-latest
             python: 3.6
             arch: x64
@@ -57,6 +53,7 @@ jobs:
             python: 3.9-dev
             arch: x64
             max-cxx-std: 17
+            args: "-DPYBIND11_FINDPYTHON=ON"
 
         exclude:
             # Currently 32bit only, and we build 64bit
@@ -92,8 +89,7 @@ jobs:
         python-version: ${{ matrix.python }}
         architecture: ${{ matrix.arch }}
 
-    - name: Setup Boost
-      if: runner.os != 'macOS'
+    - name: Setup Boost (Windows / Linux latest)
       run: echo "::set-env name=BOOST_ROOT::$BOOST_ROOT_1_72_0"
 
     - name: Update CMake