Add packaging.tags.interpreter_tag

[FFY00]

Similarly to #610, the following logic could be exposed as public API.

packaging/packaging/tags.py

Lines 501 to 506 in 606c71a

	if interp_name == "pp":
	interp = "pp3"
	elif interp_name == "cp":
	interp = "cp" + interpreter_version(warn=warn)
	else:
	interp = None

[brettcannon]

Same question as in #610; all potential tags or just the most strict one?