ci(dependabot): bump aganders3/headless-gui from 1.2 to 2.1

Bumps [aganders3/headless-gui](https://github.com/aganders3/headless-gui) from 1.2 to 2.1.
- [Release notes](https://github.com/aganders3/headless-gui/releases)
- [Commits](aganders3/headless-gui@v1.2...v2.1)

---
updated-dependencies:
- dependency-name: aganders3/headless-gui
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/ci.yml

@@ -55,7 +55,7 @@ jobs:
           pip install ${{ matrix.backend }}
 
       - name: Test
-        uses: aganders3/headless-gui@v1.2
+        uses: aganders3/headless-gui@v2.1
         with:
           run: python -m pytest