Paiwei retry flaky admin test and fix job status

Summary:
- cast and return PyObject python_task directly in add_task* methods
- job_status was not working proper with PyArg_ParseTuple when parsing python string to c char pointer; however, it works fine with PyArg_ParseTupleAndKeyword.
- fix examples/test_client.py to test updating numerator and denomiator for background jobs
- auto-generate requirements-dev.txt using 'pip freeze'
- retry flaky test_admin_client.py unittest
- set shorter timeout to speedup unittests