apibuild: Fix -refs.xml building

Another usage of deprecated 'string' functions. We are just trying to
match ascii letters here, so use a simple regex. And again drop the
aggressive exception handling, it doesn't seem to trigger for anything
in libvirt code.
Signed-off-by: NCole Robinson <crobinso@redhat.com>

docs/apibuild.py

@@ -11,7 +11,6 @@
 from __future__ import print_function
 
 import os, sys
-import string
 import glob
 import re
 
@@ -2092,14 +2091,13 @@ class docBuilder:
         str = str.replace(';', ' ')
         tokens = str.split()
         for token in tokens:
-            try:
             c = token[0]
-                if string.letters.find(c) < 0:
+            if not re.match(r"[a-zA-Z]", c):
                 pass
             elif len(token) < 3:
                 pass
             else:
-                    lower = string.lower(token)
+                lower = token.lower()
                 # TODO: generalize this a bit
                 if lower == 'and' or lower == 'the':
                     pass
@@ -2107,8 +2105,6 @@ class docBuilder:
                     self.xref[token].append(id)
                 else:
                     self.xref[token] = [id]
-            except:
-                pass
 
     def analyze(self):
         if not quiet: