Clean up include file loading code

[jtwb]

YAML include files are loaded fully into memory before parsing them with the streaming YAML parser. This is not advantageous in any way except that it makes the many tests in YamlPolicyTermTest fairly compact. It would be better to pass a file stream directly to the YAML loader.

aerleon/aerleon/lib/yaml.py

Lines 333 to 337 in 3085a51

	def _LoadIncludeFile(base_dir, inc_filename):
	"""Open an include file."""
	with open(pathlib.Path(base_dir).joinpath(inc_filename), 'r') as include_file:
	return include_file.read()

See also: #208 (comment)