Make missing long_description check more flexible.

A missing `long_description` results in `UNKNOWN` being written to
PKG-INFO, but at some point, the number of trailing newlines changed.

See
pypa/readme_renderer#231 (comment)

tests/test_check.py

@@ -107,10 +107,11 @@ def test_check_passing_distribution_with_none_renderer(
     assert capsys.readouterr().out == "Checking dist/dist.tar.gz: PASSED\n"
 
 
-def test_check_no_description(monkeypatch, capsys, caplog):
+@pytest.mark.parametrize("description", [None, "UNKNOWN\n\n", "UNKNOWN\n\n\n"])
+def test_check_no_description(description, monkeypatch, capsys, caplog):
     package = pretend.stub(
         metadata_dictionary=lambda: {
-            "description": None,
+            "description": description,
             "description_content_type": None,
         }
     )

twine/commands/check.py

@@ -93,7 +93,7 @@ def _check_file(
     content_type, params = cgi.parse_header(description_content_type)
     renderer = _RENDERERS.get(content_type, _RENDERERS[None])
 
-    if description in {None, "UNKNOWN\n\n\n"}:
+    if description is None or description.rstrip("\n") == "UNKNOWN":
         warnings.append("`long_description` missing.")
     elif renderer:
         rendering_result = renderer.render(