Ports: Add xz to Python dependencies

This makes it build the `_lzma` module, and thus makes `lzma` work.

Thanks to hjalves for the suggestion!

Ports/python3/package.sh

@@ -26,6 +26,7 @@ depends=(
     'readline'
     'sqlite'
     'termcap'
+    'xz'
     'zlib'
 )
 configopts=(