Preserve comments on list items

[jacebrowning]

This test demonstrates the expected functionality:

datafiles/tests/test_saving.py

Lines 408 to 450 in a51507b

	def with_comments_on_list_items(expect):
	sample = SampleWithListOfDataclasses()
	write(
	'tmp/sample.yml',
	"""
	# Header
	items: # Subheader
	# Section
	- name: a # Item
	score: 1
	# Section
	- name: b # Item
	score: 2
	""",
	)
	sample.datafile.load()
	sample.items[1].name = 'c'
	sample.datafile.save()
	expect(read('tmp/sample.yml')) == dedent(
	"""
	# Header
	items: # Subheader
	# Section
	- name: a # Item
	score: 1
	# Section
	- name: c # Item
	score: 2
	"""
	)

This appears to be an issue with the YAML library: https://sourceforge.net/p/ruamel-yaml/tickets/318/