bundle2: fixed usage of an attribute that was removed in py3k

bundle2: fixed usage of an attribute that was removed in py3k

Differential Revision: https://phab.mercurial-scm.org/D482

--- a/mercurial/bundle2.py	Sun Aug 13 19:58:45 2017 +0530
+++ b/mercurial/bundle2.py	Wed Aug 23 01:09:08 2017 +0000
@@ -587,7 +587,7 @@
         """add a stream level parameter"""
         if not name:
             raise ValueError('empty parameter name')
-        if name[0] not in string.letters:
+        if name[0] not in pycompat.bytestr(string.ascii_letters):
             raise ValueError('non letter first character: %r' % name)
         self._params.append((name, value))
 
@@ -765,7 +765,7 @@
         """
         if not name:
             raise ValueError('empty parameter name')
-        if name[0] not in string.letters:
+        if name[0] not in pycompat.bytestr(string.ascii_letters):
             raise ValueError('non letter first character: %r' % name)
         try:
             handler = b2streamparamsmap[name.lower()]