Ensure support for Python 3.11/3.12 (#1540)

These changes:
  * enable testing for Python 3.11/3.12 on CI
  * update classifiers at setup.py

.github/workflows/tests.yml

@@ -25,11 +25,17 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.7, 3.8, 3.9, "3.10"]
+        python-version:
+          - '3.12'
+          - '3.11'
+          - '3.10'
+          - '3.9'
+          - '3.8'
+          - '3.7'
         pyopenssl: [0, 1]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.python-version }}