Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/readthedocs.yml

@@ -11,7 +11,7 @@ jobs:
 
         steps:
           - name: Checkout repository
-            uses: actions/checkout@v2
+            uses: actions/checkout@v4
 
           - name: Set up Python
             uses: actions/setup-python@v2