Python-server workflow fixes + pytest (#17060)

* Fix referenced path for tests

* Explicit dependencies

Flask 2.3 has breaking changes

* Marks all async tests with pytest.mark.asyncio

Otherwise they are skipped by pytest and coverage is zero

* Fix path for pytest

On suggestion from https://github.com/OpenAPITools/openapi-generator/pull/17060#discussion_r1392666369

samples/server/petstore/python-aiohttp-srclayout/test_python3.sh

@@ -23,7 +23,7 @@ pip install -r $REQUIREMENTS_FILE | tee -a $REQUIREMENTS_OUT
 tox || exit 1
 
 ### static analysis of code
-flake8 --show-source petstore_api/
+flake8 --show-source ./src
 
 ### deactivate virtualenv
 if [ $DEACTIVE == true ]; then