Update the dependency on actions/checkout.

The new version uses Node.js 16 (eliminating warnings about Node.js 12
deprecation).

.github/workflows/tests.yml

@@ -32,7 +32,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     continue-on-error: ${{ matrix.experimental }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}