Bump actions/checkout from 3 to 4 (#5)

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -3,7 +3,7 @@ name: CI
 on:
   pull_request:
   push:
-    branches: [main, signal/gh]
+    branches: [main]
     tags: [v*.*.*]
 
 jobs:
@@ -25,9 +25,11 @@ jobs:
           - python-version: '2.7'
             container-image: python:2.7
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
-          fetch-depth: 0 # fetch all history for setuptools_scm to be able to read tags
+          # Fetch all the history so setuptools_scm can version relative to the
+          # most recent version tag.
+          fetch-depth: 0
 
       - uses: actions/setup-python@v4
         # Only set up Python if we're running directly on an agent. If we're in