Update CI Python versions

Python 3.7 is past end-of-life, and even 3.8 is pretty outdated. Switch to testing on 3.8 (oldest supported) and 3.12 (current release).

.github/workflows/main.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [3.7, 3.8]
+        python-version: [3.8, 3.12]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
@@ -29,4 +29,4 @@ jobs:
         name: examples-and-tutorials
         path: |
           examples/
-          tutorial/
+          tutorial/