Restore support for older BeautifulSoup4 versions

.travis.yml

@@ -8,9 +8,6 @@ jobs:
   include:
     - python: "3.5"
       env: TOXENV=oldbs4
-  allow_failures:
-    - python: "3.5"
-      env: TOXENV=oldbs4
 # command to install dependencies
 addons:
   apt:

linkcheck/HtmlParser/htmlsax.py

@@ -93,7 +93,10 @@ class Parser(object):
                             self.handler.comment(comment)
             elif isinstance(content, Doctype):
                 if hasattr(self.handler, 'doctype'):
-                    self.handler.doctype(content)
+                    self.handler.doctype(
+                        content[len('DOCTYPE '):]
+                        if content.upper().startswith('DOCTYPE ')
+                        else content)
             elif isinstance(content, Comment):
                 if hasattr(self.handler, 'comment'):
                     self.handler.comment(content.strip())