html2text: fix syntax error for python 3.12

See https://docs.python.org/3/whatsnew/3.12.html
"A backslash-character pair that is not a valid escape sequence now generates a SyntaxWarning"
This was getting elevated to an error inside readme2modulestrings.py,
causing failure to generate module_strings.h

modules/html2text.py

@@ -627,7 +627,7 @@ def o(self, data, puredata=0, force=0):
                     self.drop_white_space = 0
 
             if puredata and not self.pre:
-                data = re.sub('\s+', ' ', data)
+                data = re.sub('\\s+', ' ', data)
                 if data and data[0] == ' ':
                     self.space = 1
                     data = data[1:]