CI: Add Python 3.12 tests, unpin the flake8 Python version

Author: effigies

.github/workflows/pythonpackage.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.8', '3.9',  '3.10',  '3.11']
+        python-version: ['3.8', '3.9',  '3.10',  '3.11', '3.12']
 
     steps:
     - name: Set up Python ${{ matrix.python-version }}
@@ -91,8 +91,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - name: Set up Python 3.7
+    - name: Set up Python 3
       uses: actions/setup-python@v4
-      with:
-        python-version: 3.7
     - run: pipx run flake8 nitransforms