add sort_by_generation to query_step; support get sub steps of a specific step; fix hooks of task #103

	name: Python unit-tests

	on:
	- pull_request

	jobs:
	build:
	runs-on: ubuntu-latest

	steps:
	- uses: actions/checkout@master
	- name: Set up Python 3.8
	uses: actions/setup-python@master
	with:
	python-version: 3.8
	- name: Install dependencies
	run: \|
	pip install .
	pip install pytest
	- name: Test
	run: DFLOW_MODE=debug pytest -vs tests

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