Restore Python 2 compatibility in Python bindings.

Fixes:

failed to find and generate stubs for libxml2, aborting ...
(<type 'exceptions.SyntaxError'>, SyntaxError('invalid syntax', ('/opt/local/var/macports/build/py27-libxml2-116ed89d/work/libxml2-2.15.2/python/generator.py', 360, 61, "        raise Exception(f'Doxygen XML not found in {dstPref}')\n")))

failed to find and generate stubs for libxml2, aborting ...
(<type 'exceptions.LookupError'>, LookupError('unknown encoding: unicode',))

This patch was created with the awk and sed scripts posted here:

https://trac.macports.org/ticket/73605#comment:11
--- generator.py.orig	2026-03-04 03:38:57.000000000 -0600
+++ generator.py	2026-03-28 10:11:33.000000000 -0500
@@ -357,7 +357,7 @@
 if not os.path.isdir(xmlDocDir):
     xmlDocDir = dstPref + '/doc/xml'
     if not os.path.isdir(xmlDocDir):
-        raise Exception(f'Doxygen XML not found in {dstPref}')
+        raise Exception('Doxygen XML not found in %s' % dstPref)
 
 def extractDocs(node):
     text = ''
@@ -416,13 +416,13 @@
                 docs = extractDocs(func.find('detaileddescription'))
 
                 rtype = etree.tostring(func.find('type'),
-                    method='text', encoding='unicode').rstrip()
+                    method='text', encoding='utf-8').rstrip()
 
                 valid = True
                 args = []
                 for arg in func.findall('param'):
                     atype = etree.tostring(arg.find('type'),
-                        method='text', encoding='unicode').rstrip()
+                        method='text', encoding='utf-8').rstrip()
                     if atype == 'void':
                         continue
 
@@ -440,9 +440,9 @@
 
                 cond = None
                 if module1 != '':
-                    cond = f'defined(LIBXML_{module1}_ENABLED)'
+                    cond = 'defined(LIBXML_%s_ENABLED)' % module1
                 if module2 != '':
-                    cond += f' && defined(LIBXML_{module2}_ENABLED)'
+                    cond += ' && defined(LIBXML_%s_ENABLED)' % module2
 
                 functions[name] = (docs, [rtype], args, module, cond)
         elif kind == 'enum':
@@ -461,7 +461,7 @@
                     else:
                         evalue = init.text.lstrip()
                         if evalue[0] != '=':
-                            raise Exception(f'invalid init value {init}')
+                            raise Exception('invalid init value %s' % init)
                         evalue = eval(evalue[1:].strip())
 
                     edict[ename] = evalue