Add a "cpNNN-none-any" tag (#541)

Closes #511

See pypa/pip#10923 for motivation.

Co-authored-by: Brett Cannon <brett@python.org>

packaging/tags.py

@@ -499,6 +499,9 @@ def sys_tags(*, warn: bool = False) -> Iterator[Tag]:
         yield from generic_tags()
 
     if interp_name == "pp":
-        yield from compatible_tags(interpreter="pp3")
+        interp = "pp3"
+    elif interp_name == "cp":
+        interp = "cp" + interpreter_version(warn=warn)
     else:
-        yield from compatible_tags()
+        interp = None
+    yield from compatible_tags(interpreter=interp)

tests/test_tags.py

@@ -1226,3 +1226,16 @@ def test_pypy_first_none_any_tag(self, monkeypatch):
                 break
 
         assert tag == tags.Tag("pp3", "none", "any")
+
+    def test_cpython_first_none_any_tag(self, monkeypatch):
+        # When building the complete list of cpython tags, make sure the first
+        # <interpreter>-none-any one is cpxx-none-any
+        monkeypatch.setattr(tags, "interpreter_name", lambda: "cp")
+
+        # Find the first tag that is ABI- and platform-agnostic.
+        for tag in tags.sys_tags():
+            if tag.abi == "none" and tag.platform == "any":
+                break
+
+        interpreter = f"cp{tags.interpreter_version()}"
+        assert tag == tags.Tag(interpreter, "none", "any")