Make lzma.{encode,decode}_filter_properties private.

These functions were originally added to support LZMA compression in the zipfile
module, and are not of interest for the majority of users.

They can be made public in 3.4 if there is user interest, but in the meanwhile,
I've opted to present a smaller, simpler API for the module's initial release.
diff --git a/Lib/lzma.py b/Lib/lzma.py
index f623204..ffcf812 100644
--- a/Lib/lzma.py
+++ b/Lib/lzma.py
@@ -19,12 +19,12 @@
 
     "LZMACompressor", "LZMADecompressor", "LZMAFile", "LZMAError",
     "open", "compress", "decompress", "is_check_supported",
-    "encode_filter_properties", "decode_filter_properties",
 ]
 
 import builtins
 import io
 from _lzma import *
+from _lzma import _encode_filter_properties, _decode_filter_properties
 
 
 _MODE_CLOSED   = 0