Fix typo: a -> addr. Found by pychecker.

src/ZEO/zrpc/client.py

@@ -65,7 +65,8 @@ class ConnectionManager:
             for addr in addrs:
                 addr_type = self._guess_type(addr)
                 if addr_type is None:
-                    raise ValueError, "unknown address in list: %s" % repr(a)
+                    raise ValueError, (
+                        "unknown address in list: %s" % repr(addr))
                 addrlist.append((addr_type, addr))
             return addrlist