Add usage command and allow running the emulator with no serial conne… #204

	# from https://docs.github.com/en/actions/guides/building-and-testing-python

	name: Python syntax checker

	on: [push]

	jobs:
	build:

	runs-on: ubuntu-latest
	strategy:
	matrix:
	# version range and latest minor release (possibly 3.9.1)
	python-version: [3.7, 3.8, '3.x']

	steps:
	- uses: actions/checkout@v2
	- name: Set up Python ${{ matrix.python-version }}
	uses: actions/setup-python@v2
	with:
	python-version: ${{ matrix.python-version }}
	- name: Install dependencies
	run: \|
	python -m pip install --upgrade pip
	pip install flake8
	# next line is alternative to the previous one, for future use of pytest
	# pip install flake8 pytest
	# next line is for future use of requirements file
	# if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
	- name: Lint with flake8
	run: \|
	# stop the build if there are Python syntax errors or undefined names
	flake8 . --count --select=E9,F63,F7,F72,F82 --show-source --statistics
	# next lines is for future use of more accurate statistics
	# exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
	# flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
	# next lines is for future use of pytest
	#- name: Test with pytest
	# run: \|
	# pytest

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