diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3b4b761a..16259201 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,9 +11,9 @@ jobs:
   test:
     name: "Python ${{ matrix.python-version }}"
     runs-on: sfdc-ubuntu-latest
-    strategy:
-      matrix:
-        python-version: ["3.8", "3.9", "3.10"]
+    strategy: # when github action adds 3.12, you can add it here.
+      matrix: # https://github.com/actions/python-versions/releases
+        python-version: ["3.8", "3.9", "3.11"]
     steps:
       - uses: "actions/checkout@v2"
       - uses: "actions/setup-python@v1"
@@ -97,7 +97,7 @@ jobs:
     runs-on: sfdc-ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.8", "3.10"]
+        python-version: ["3.8", "3.11"]
     steps:
       - uses: "actions/checkout@v2"
       - uses: "actions/setup-python@v1"
@@ -122,7 +122,7 @@ jobs:
     runs-on: sfdc-windows-latest
     strategy:
       matrix:
-        python-version: ["3.9", "3.10"]
+        python-version: ["3.8", "3.11"]
     steps:
       - uses: "actions/checkout@v2"
       - uses: "actions/setup-python@v1"
@@ -133,7 +133,11 @@ jobs:
         run: |
           python -VV
           python -m pip install --upgrade pip
-          pip install -r requirements_dev.txt
+          make dev-install
+          # Snowfakery should work both with and without graphviz.
+          # On Linux we test without it and on Windows we test with it.
+          # The Unit tests adapt to both situations
+          choco install graphviz
 
       - name: Run Tests
         run: python -m pytest
diff --git a/tests/test_images.py b/tests/test_images.py
index 8df0fe30..538005b3 100644
--- a/tests/test_images.py
+++ b/tests/test_images.py
@@ -130,6 +130,30 @@ def graphviz_available():
 
 
 class TestImageOuputStreams:
+    @mock.patch("subprocess.Popen")
+    def test_image_outputs_mocked(self, popen):
+        png = "out.png"
+        svg = "out.svg"
+        txt = "out.txt"
+        dot = "out.dot"
+        popen.return_value.communicate = lambda: (mock.Mock(), mock.Mock())
+        generate_cli.main(
+            [
+                str(sample_yaml),
+                "--output-file",
+                png,
+                "--output-file",
+                svg,
+                "--output-file",
+                txt,
+                "--output-file",
+                dot,
+            ],
+            standalone_mode=False,
+        )
+        for call in popen.mock_calls:
+            assert call[1][0][0] == "dot"
+
     def test_image_outputs(self):
         if not graphviz_available():
             pytest.skip("Graphviz is not installed")
@@ -153,7 +177,7 @@ def test_image_outputs(self):
                 ],
                 standalone_mode=False,
             )
-            assert png.read_bytes().startswith(b"\x89PNG\r\n")
+            assert png.read_bytes().startswith(b"\x89PNG"), png.read_bytes()[0:10]
             assert svg.read_bytes().startswith(b"<?xml"), svg.read_bytes()[0:5]
             assert svg.read_bytes().startswith(b"<?xml"), svg.read_bytes()[0:5]
             assert dot.read_bytes().startswith(b"/* Ge"), dot.read_bytes()[0:5]
diff --git a/tox.ini b/tox.ini
index c3a38b41..99dd0f97 100644
--- a/tox.ini
+++ b/tox.ini
@@ -16,6 +16,9 @@ install_command =
 python =
     3.8: py38
     3.9: py39
+    3.10: py310
+    3.11: py311
+    3.12: py312
 
 [testenv:lint]
 description = Run all pre-commit hooks.