Bump actions/checkout from 2 to 3 (#19)

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/tests.yml

@@ -21,7 +21,7 @@ jobs:
 
         steps:
             - name: Checkout code
-              uses: actions/checkout@v2
+              uses: actions/checkout@v3
 
             - name: Setup PHP
               uses: shivammathur/setup-php@v2