Drop support for Python 3.6 on macOS

Python 3.6 has been EOL for a while now, so not worth spending time on
fixing the failing build for 3.6 on MacOS.

.github/workflows/wheels.yaml

@@ -46,7 +46,7 @@ jobs:
           CIBW_ENVIRONMENT_WINDOWS: >
             CMAKE_GENERATOR="${{ matrix.cmake_generator }}"
             CMAKE_GENERATOR_PLATFORM="${{ matrix.cmake_generator_platform }}"
-          CIBW_SKIP: pp* *-musllinux_*
+          CIBW_SKIP: pp* *-musllinux_* cp36*-macos*
           CIBW_ARCHS: ${{ matrix.arch }}
         run: |
             python -m cibuildwheel --output-dir wheelhouse python/