Install setuptools alongside tox-factor

Ensure it is available for creating the sdist on Python 3.12+.

#400 (comment)

Author: musicinmybrain

.github/workflows/UnitTesting.yaml

@@ -41,7 +41,7 @@ jobs:
           python-version: ${{ env[matrix.python-version] }}
 
       - name: Install tox
-        run: pip install "tox<=3.27.1" -U tox-factor
+        run: pip install "tox<=3.27.1" -U tox-factor setuptools
 
       - name: Cache tox environment
         # Preserves .tox directory between runs for faster installs