ci: Python tests: Temporarily hardcode Python version to 3.7

It's needed to fix CI until https://github.com/breathe-doc/breathe/issues/943 is done.

.github/workflows/ci.yml

@@ -205,10 +205,12 @@ jobs:
           name: ${{ matrix.os }}-libdeltachat.a
           path: target/debug
 
+      # TODO: Temporarily hardcode Python version to 3.7 until
+      # https://github.com/breathe-doc/breathe/issues/943 is done.
       - name: Install python
         uses: actions/setup-python@v4
         with:
-          python-version: ${{ matrix.python }}
+          python-version: 3.7
 
       - name: Install tox
         run: pip install tox

deltachat-rpc-client/tests/test_something.py

@@ -363,7 +363,7 @@ async def test_import_export(acfactory, tmp_path) -> None:
 
 def test_openrpc_command_line() -> None:
     """Test that "deltachat-rpc-server --openrpc" command returns an OpenRPC specification."""
-    out = subprocess.run(["deltachat-rpc-server", "--openrpc"], capture_output=True).stdout
+    out = subprocess.run(["deltachat-rpc-server", "--openrpc"], capture_output=True, check=True).stdout
     openrpc = json.loads(out)
     assert "openrpc" in openrpc
     assert "methods" in openrpc