Bump actions/checkout from 3 to 4 (#137)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

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

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

.github/workflows/draft-pdf.yml

@@ -10,7 +10,7 @@ jobs:
     name: Paper Draft
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Build draft PDF
         uses: openjournals/openjournals-draft-action@master
         with:

.github/workflows/unit_tests.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup Python 3.9
         uses: actions/setup-python@v4
         with:
@@ -71,7 +71,7 @@ jobs:
         shell: bash
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup Python ${{ matrix.python-version }}
         uses: actions/setup-python@v4
         with:
@@ -121,7 +121,7 @@ jobs:
         shell: bash
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           path: main # clone repository in a sub-directory
       - name: Setup Python ${{ matrix.python-version }}
@@ -179,7 +179,7 @@ jobs:
     if: startsWith(github.ref, 'refs/tags/')
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup Python ${{ matrix.python-version }}
         uses: actions/setup-python@v4
         with: