add back Python 3.6 build on CIBUILDWHEEL

Author: jmrohwer

.github/workflows/cibuildwheel.yml

@@ -53,10 +53,10 @@ jobs:
           CIBW_BUILD: "cp38-*"
           CIBW_BEFORE_BUILD: pip install numpy~=1.17.0
 
-      - name: Build wheels for CPython 3.7
+      - name: Build wheels for CPython 3.6 and 3.7
         uses: pypa/[email protected]
         env:
-          CIBW_BUILD: "cp37-*"
+          CIBW_BUILD: "cp36-* cp37-*"
           CIBW_BEFORE_BUILD: pip install numpy~=1.15.0
 
       - uses: actions/upload-artifact@v2