ros2 pkg create for ament_python packages should include xmllint

[sloretz]

Feature request

ament/ament_cmake#553

Feature description

New ament_python packages should include an xmllint test to match the default linters given to ament_cmake packages

Implementation considerations

Copy this file:

https://github.com/ros2/ros2cli/blob/rolling/ros2pkg/test/test_xmllint.py

Here:

https://github.com/ros2/ros2cli/tree/rolling/ros2pkg/ros2pkg/resource/ament_python

Make sure test/test_xmllint.py is created in new packages here:

ros2cli/ros2pkg/ros2pkg/api/create.py

Lines 173 to 188 in a3198b8

	test_directory = _create_folder('test', package_directory)
	_create_template_file('ament_python',
	'test_copyright.py.em',
	test_directory,
	'test_copyright.py',
	{})
	_create_template_file('ament_python',
	'test_flake8.py.em',
	test_directory,
	'test_flake8.py',
	{})
	_create_template_file('ament_python',
	'test_pep257.py.em',
	test_directory,
	'test_pep257.py',
	{})

Make sure the package.xml gets a <test_depend> on ament_xmllint here:

ros2cli/ros2pkg/ros2pkg/verb/create.py

Lines 140 to 141 in a3198b8

	test_dependencies = ['ament_copyright', 'ament_flake8', 'ament_pep257',
	'python3-pytest']

Bonus: create ament_python equivalent tests with assertion that test/test_xmllint.py exists here:

ros2cli/ros2pkg/test/test_cli.py

Line 122 in a3198b8

def test_create_package(self):