Allow 'long_description_content_type' warnings for new versions of packaging. Fixes #1858.

changelog.d/1858.misc.rst

+Fixed failing integration test triggered by 'long_description_content_type' in packaging.

setuptools/tests/test_integration.py

@@ -143,6 +143,7 @@ def test_build_deps_on_distutils(request, tmpdir_factory, build_dep):
             'tests_require',
             'python_requires',
             'install_requires',
+            'long_description_content_type',
         ]
         assert not match or match.group(1).strip('"\'') in allowed_unknowns