fix byte-array / str

python3 returns byte-arrays instead of strings for ldap-data in
ContextCSNs, which makes the listcomprehension fail with a TypeError
when using the --serverID flag like so:

    CSN = [s for s in contextCSNs if sub in s]
TypeError: a bytes-like object is required, not 'str'

explicitly encoding 'sub' to byte-array, not sure if this is best way,
but seems to work well enough.

check_syncrepl_extended

@@ -323,7 +323,7 @@ class LdapServer(object):
             if serverid is False:
                 return contextCSNs[0]
             csnid = str(format(serverid, 'X')).zfill(3)
-            sub = '#%s#' % csnid
+            sub = str.encode('#%s#' % csnid, encoding="ascii", errors="replace")
             CSN = [s for s in contextCSNs if sub in s]
             if not CSN:
                 logging.error(