Upadate tested Python versions on ci

- Remove 3.7, add 3.11 to match in-toto
- Only test on 3.11 on macos, for the same reasons as
  theupdateframework/python-tuf#2618

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

.github/workflows/ci.yml

@@ -16,8 +16,11 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ['3.7', '3.8', '3.9', '3.10']
-        os: [ubuntu-latest, macos-latest]
+        python-version: ['3.8', '3.9', '3.10', '3.11']
+        os: [ubuntu-latest]
+        include:
+          - python-version: "3.11"
+            os: macos-latest
 
     runs-on: ${{ matrix.os }}
     steps: