Use fully_normalize_name() instead of str.lower()

sphinx/domains/std.py

@@ -14,6 +14,7 @@ import unicodedata
 
 from six import iteritems
 from docutils import nodes
+from docutils.nodes import fully_normalize_name
 from docutils.parsers.rst import directives
 from docutils.statemachine import ViewList
 
@@ -636,7 +637,7 @@ class StandardDomain(Domain):
                 return None
 
             title = contnode.astext()
-            if target == title.lower():
+            if target == fully_normalize_name(title):
                 prefix = env.config.numfig_format.get(figtype, '')
                 title = prefix.replace('%s', '#')
                 newtitle = prefix % '.'.join(map(str, fignumber))