Compatibility with Jinja2 3.1.0 (#2800)

mkdocs/utils/filters.py

@@ -1,9 +1,12 @@
-import jinja2
+try:
+    from jinja2 import pass_context as contextfilter
+except ImportError:
+    from jinja2 import contextfilter
 
 from mkdocs.utils import normalize_url
 
 
-@jinja2.contextfilter
+@contextfilter
 def url_filter(context, value):
     """ A Template filter to normalize URLs. """
     return normalize_url(value, page=context['page'], base=context['base_url'])