tests: Skip readme renderer on py 3.9

Readme renderer uses html5lib, which does not work on Python 3.9.

See discussion at:
  html5lib/html5lib-python#419

tests/test_readme.py

@@ -1,12 +1,20 @@
+import sys
 from io import StringIO
 
 try:
     from pathlib import Path
 except ImportError:
     from pathlib2 import Path
-from readme_renderer.rst import render
 
+import pytest
 
+if sys.version_info < (3, 9):
+    from readme_renderer.rst import render
+else:
+    render = None
+
+
+@pytest.mark.skipif(render is None, reason="html5lib issue #419")
 def test_pypi_rendering():
     # Adapted from `https://stackoverflow.com/questions/46766570/`.
     readme = Path("README.rst")