Upgrade GitHub Actions checkout (#94)

Also, upgrade from `pypy3.9` to `pypy3.10` and remove the `continue-on-error` for Python 3.12.

As recommended at jaraco/cssutils#41

.github/workflows/main.yml

@@ -36,12 +36,12 @@ jobs:
           platform: ubuntu-latest
         - python: "3.10"
           platform: ubuntu-latest
-        - python: pypy3.9
+        - python: pypy3.10
           platform: ubuntu-latest
     runs-on: ${{ matrix.platform }}
-    continue-on-error: ${{ matrix.python == '3.12' }}
+    continue-on-error: ${{ matrix.python == '3.13' }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Python
         uses: actions/setup-python@v4
         with:
@@ -56,7 +56,7 @@ jobs:
   diffcov:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
       - name: Setup Python
@@ -76,7 +76,7 @@ jobs:
     env:
       TOXENV: docs
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Python
         uses: actions/setup-python@v4
       - name: Install tox
@@ -109,7 +109,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Setup Python
         uses: actions/setup-python@v4
         with: