Support compresslevel when writing xz files with the lzma module
Support compresslevel when writing xz files with the lzma module

lzma.open()'s preset kwarg is a 0-9 integer just like gzip.open's
compresslevel kwarg.  The default for the stdlib and `xz` command is 6,
but be explicit about it since 6 is what xopen() documents and what is
used for gzip too.

marcelm/xopen