Try on mac

Author: tomwhite

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
   build:
     # Scheduled runs only on the origin org
     if: (github.event_name == 'schedule' && github.repository_owner == 'pystatgen') || (github.event_name != 'schedule')
-    runs-on: ubuntu-18.04
+    runs-on: macos-latest
     strategy:
       matrix:
         python-version: [3.7, 3.8, 3.9]