url unquote the path before checking if it refers to a CGI script (closes #21766)
diff --git a/Lib/http/server.py b/Lib/http/server.py
index bcfe894..1f4d1bb 100644
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -946,7 +946,7 @@
         (and the next character is a '/' or the end of the string).
 
         """
-        collapsed_path = _url_collapse_path(self.path)
+        collapsed_path = _url_collapse_path(urllib.parse.unquote(self.path))
         dir_sep = collapsed_path.find('/', 1)
         head, tail = collapsed_path[:dir_sep], collapsed_path[dir_sep+1:]
         if head in self.cgi_directories: