www/trac-devel: fix build with a recent Python version, patch adopted from the upstream

www/trac-devel/files/patch-trac_util_html.py

 --- trac/util/html.py.orig	2021-05-09 23:08:49 UTC
 +++ trac/util/html.py
-@@ -24,7 +24,7 @@ import sys
- from html import entities
+@@ -25,6 +25,10 @@ from html import entities
  from html.parser import HTMLParser
  
--from markupsafe import Markup, escape as escape_quotes
-+from markupsafe import Markup, escape as escape_quotes, soft_unicode
+ from markupsafe import Markup, escape as escape_quotes
++try:
++    from markupsafe import soft_str as soft_unicode
++except ImportError:
++    from markupsafe import soft_unicode
  
  try:
      from babel.support import LazyProxy