Set up aliases in campbells.formatter with a classmethod

[lmmx]

At the end of formatter.py, aliases are set using the REGISTRY class var and the __init__ methods of the Formatter subclasses HTMLFormatter and XMLFormatter:

campbells/src/campbells/formatter.py

Lines 160 to 199 in bf269f7

	class HTMLFormatter(Formatter):
	"""A generic Formatter for HTML."""
	REGISTRY = {}
	def __init__(self, *args, **kwargs):
	super().__init__(self.HTML, *args, **kwargs)
	class XMLFormatter(Formatter):
	"""A generic Formatter for XML."""
	REGISTRY = {}
	def __init__(self, *args, **kwargs):
	super().__init__(self.XML, *args, **kwargs)
	# Set up aliases for the default formatters.
	HTMLFormatter.REGISTRY["html"] = HTMLFormatter(
	entity_substitution=EntitySubstitution.substitute_html,
	)
	HTMLFormatter.REGISTRY["html5"] = HTMLFormatter(
	entity_substitution=EntitySubstitution.substitute_html,
	void_element_close_prefix=None,
	empty_attributes_are_booleans=True,
	)
	HTMLFormatter.REGISTRY["minimal"] = HTMLFormatter(
	entity_substitution=EntitySubstitution.substitute_xml,
	)
	HTMLFormatter.REGISTRY[None] = HTMLFormatter(entity_substitution=None)
	XMLFormatter.REGISTRY["html"] = XMLFormatter(
	entity_substitution=EntitySubstitution.substitute_html,
	)
	XMLFormatter.REGISTRY["minimal"] = XMLFormatter(
	entity_substitution=EntitySubstitution.substitute_xml,
	)
	XMLFormatter.REGISTRY[None] = Formatter(
	Formatter(Formatter.XML, entity_substitution=None),
	)

This is not a great pattern, instead use a classmethod to make it clearer what is in common (this section is hard to read)