fix: temporary constrain fastapi version (#418)

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

tests/test_fastapi_endpoints.py

@@ -54,6 +54,14 @@ async def test_health(client: AsyncClient):
     assert response.json() == {"status": "ok"}
 
 
+@pytest.mark.asyncio
+async def test_openapijson(client: AsyncClient):
+    response = await client.get("/openapi.json")
+    assert response.status_code == 200
+    schema = response.json()
+    assert "openapi" in schema
+
+
 @pytest.mark.asyncio
 async def test_convert_file(client: AsyncClient, auth_headers: dict):
     """Test convert single file to all outputs"""