ci: add wheels for Xcode's arm64 python 3.8

Upstream python only support arm64 starting from python 3.9, but Xcode
ships with an arm64 python 3.8. As cibuildwheel doesn't support using
Xcode's python, handle this case manually.

.github/workflows/wheel.yml

@@ -44,6 +44,8 @@ jobs:
           - 'cp39-*'
         arch: ['x86_64']
         include:
+          - py: 'cp38-*'
+            arch: 'arm64'
           - py: 'cp39-*'
             arch: 'arm64'
     runs-on: 'macos-10.15'
@@ -57,16 +59,35 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: '3.8'
+        if: ${{ matrix.py != 'cp38-*' || matrix.arch != 'arm64' }}
 
       - uses: actions/checkout@v2
 
       - name: Install Dependencies
         run: |
           pip install --require-hashes -r ci/requirements.macoswheels.txt
+        if: ${{ matrix.py != 'cp38-*' || matrix.arch != 'arm64' }}
+
+      - name: Install Dependencies
+        run: |
+          /Applications/Xcode_12.2.app/Contents/Developer/usr/bin/pip3 install --user --require-hashes -r ci/requirements.macoswheels.txt
+          /Applications/Xcode_12.2.app/Contents/Developer/usr/bin/pip3 install --user wheel
+        if: ${{ matrix.py == 'cp38-*' && matrix.arch == 'arm64' }}
 
       - name: Build Wheel
         run: |
           cibuildwheel --output-dir dist
+        if: ${{ matrix.py != 'cp38-*' || matrix.arch != 'arm64' }}
+
+      - name: Build Wheel
+        run: |
+          /Applications/Xcode_12.2.app/Contents/Developer/usr/bin/python3 setup.py bdist_wheel
+        env:
+          _PYTHON_HOST_PLATFORM: 'macosx-11.0-arm64'
+          ARCHFLAGS: '-arch arm64'
+          MACOSX_DEPLOYMENT_TARGET: '11.0'
+          SDKROOT: '/Applications/Xcode_12.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk'
+        if: ${{ matrix.py == 'cp38-*' && matrix.arch == 'arm64' }}
 
       - name: Upload Wheel
         uses: actions/upload-artifact@v2