chore: add python 3.9 to unit tests

Author: jonathannorris

.github/workflows/unit_test.yml

@@ -1,14 +1,14 @@
 name: Unit Tests
 
-on: [ push ]
+on: [push]
 
 jobs:
   unit_tests:
     name: Unit Tests
     runs-on: ${{matrix.os}}
     strategy:
       matrix:
-        python-version: [ "3.12" ]
+        python-version: ["3.12", "3.9"]
         os: [ubuntu-latest, windows-latest]
 
     steps:
@@ -17,11 +17,11 @@ jobs:
         uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
-          cache: 'pip'
+          cache: "pip"
       - name: Install dependencies
         run: |
           pip install --upgrade pip
           pip install -r requirements.test.txt
       - name: Run unit tests
         run: |
-          pytest
+          pytest