Bump actions/checkout from v1 to v2.3.4 (#45)

Bumps [actions/checkout](https://github.com/actions/checkout) from v1 to v2.3.4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v1...5a4ac90)

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

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
         python-version: ["3.6", "3.7", "3.8", "3.9"]
     name: Python ${{ matrix.python-version }}
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.4
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v2.2.2
         with: