Re-land "Check in a simple pure-python based Markdown previewer."

tools/checklicenses/checklicenses.py

Index: tools/checklicenses/checklicenses.py
diff --git a/tools/checklicenses/checklicenses.py b/tools/checklicenses/checklicenses.py
index 68998d6f9e67586a5d7aea222aa69f8938fa50e9..54081745cbe2963da496aa7fce18dfd5b995a83f 100755
--- a/tools/checklicenses/checklicenses.py
+++ b/tools/checklicenses/checklicenses.py
@@ -151,6 +151,14 @@ PATH_SPECIFIC_WHITELISTED_LICENSES = {
         'GPL (v3 or later)',
         'MPL (v1.1) LGPL (unversioned/unknown version)',
     ],
+
+    # The project is BSD-licensed but the individual files do not have
+    # consistent license headers. Also, this is just used in a utility
+    # and not shipped. https://github.com/waylan/Python-Markdown/issues/435
+    'third_party/Python-Markdown': [
+        'UNKNOWN',
+    ],
+
     'third_party/WebKit': [
         'UNKNOWN',
     ],