Revert "Test in MacOS 10.15. Drop test in py37"

This reverts commit 4094cf2.

There were a bunch of other places where the macOS image name also
needed to be updated, so the tests haven't been running since the above
commit on macOS. macos-latest should be fine now anyway.

.github/workflows/ci.yml

@@ -28,8 +28,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-latest, ubuntu-latest, macos-10.15]
-        python-version: [3.8, 3.9]
+        os: [windows-latest, ubuntu-latest, macos-latest]
+        python-version: [3.7, 3.8, 3.9]
 
     steps:
     - uses: actions/checkout@v2