CI: Update version of GH checkout action (#232)

* CI: Update version of GH checkout action

Version 4 should get rid of the deprecation warning:

The `set-output` command is deprecated and will be disabled soon. Please
upgrade to using Environment Files. For more information see:
https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

* Oops, changed the wrong action's version

.github/workflows/build-test.yml

@@ -48,7 +48,7 @@ jobs:
       shell: bash
 
     - name: Set up Python ${{ matrix.python }}
-      uses: actions/setup-python@v3
+      uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python }}