#10004: in Q encoded word ignore '=xx' when xx is not valid hex.

Bug report and fix by Thomas Guettler.
diff --git a/Lib/email/quoprimime.py b/Lib/email/quoprimime.py
index 12df028..168dfff 100644
--- a/Lib/email/quoprimime.py
+++ b/Lib/email/quoprimime.py
@@ -294,4 +294,4 @@
     the high level email.header class for that functionality.
     """
     s = s.replace('_', ' ')
-    return re.sub(r'=\w{2}', _unquote_match, s, re.ASCII)
+    return re.sub(r'=[a-fA-F0-9]{2}', _unquote_match, s, re.ASCII)