GitHub concurrency

[henryiii]

If we push several times to a single PR, the GitHub Actions CI runs on each push. Some of the other CI's cancel old runs, but GHA requires a setting to do so. Should we add that setting? I think it would look like this:

diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index cd50c3e..6d5b97b 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -28,6 +28,9 @@ jobs:
         os: [ubuntu-latest, windows-latest, macos-latest]
         python_version: ['3.7']
     timeout-minutes: 180
+    concurrency:
+      group: ${{github.ref}}-${{matrix.python-version}}-${{matrix.os}}-test
+      cancel-in-progress: true
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2