Tweak Python versions in CI (#90)

* CI: test on 3.11 and 3.12 as well

* CI: pin Ubuntu version to fix Python 3.6

See actions/setup-python#544

* CI: remove 2.7 support to fix build

See actions/setup-python#672

* Bump actions/setup-python to v5

.github/workflows/build.yml

@@ -12,15 +12,15 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: ['2.7', '3.6', '3.7', '3.8', '3.9', '3.10']
+        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', '3.11', '3.12']
 
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies