ltfs_ordered_copy: accept str values from xattr.get() (#522)

[MitchelTighe]

Related to #522

The destination check calls sig.startswith(b"LTFS") on the value returned by xattr.get(). Depending on the xattr module in use, that value can be str rather than bytes, and the check then fails with:

Check destination:startswith first arg must be str, not bytes

That's the mirror image of the error reported in #522. This PR encodes a str value before the comparison. It's a 2-line change.

Why "related" rather than "fixes": the exact error in #522 (must be bytes ... not str) no longer reproduces on current main, because the check already compares against b"LTFS". This PR covers the remaining str case.

Update: an earlier version of this PR also changed the shebang to python3. I dropped that after reading #424/#425, which explain why the generic python shebang is kept and that packagers adjust it per distro.

Testing (local harness with an xattr shim that returns str, no real tape):

• before: aborts at the destination check;
• after: the copy proceeds.

Written with help from an AI assistant (Claude).

[MitchelTighe]

Closing this one myself. The error reported in #522 no longer reproduces on current main, and pyxattr's xattr.get() returns bytes, so the str case this guarded against is hypothetical. Sorry for the noise.