Use python 3.12 for mypy in ci

On python 3.13 we are getting a strange type error that does also not
happen in the nix check:

khard/khard.py:658: error: Argument "policy" to "message_from_string" has incompatible type "EmailPolicy[EmailMessage]"; expected "Policy[Message[str, str]]"  [arg-type]

.github/workflows/ci.yml

@@ -31,7 +31,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v4
       with:
-        python-version: "3.13"
+        python-version: "3.12"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip