Find suitable new location for format_tag

[braingram]

This is currently in the soon the be deprecated asdf.types:

asdf/asdf/types.py

Lines 17 to 29 in f726fe8

	def format_tag(organization, standard, version, tag_name):
	"""
	Format a YAML tag.
	"""
	tag = f"tag:{organization}:{standard}/{tag_name}"
	if version is None:
	return tag
	if isinstance(version, AsdfSpec):
	version = str(version.spec)
	return f"{tag}-{version}"

It does not depend on the legacy type system and could be moved to a new location.

I've only found 1 use of this in test_schema.py:

asdf/asdf/tests/test_schema.py

Line 639 in f726fe8

tag = types.format_tag("stsci.edu", "asdf", "1.0.0", "fits/fits")

[WilliamJamieson]

I would say either put it in testing.helpers, asdf.schema, or asdf.util.

[WilliamJamieson]

My personal preference is for the helpers as it seems to be only used for testing.