1.9.0 fseek for zstandard file results in io.UnsupportedOperation: underlying stream is not seekable

Can be easily reproduced ``` /nfs/site/home/<user>/pyenv3.11.1/bin/python3 Python 3.11.1 (main, Dec 6 2023, 09:00:14) [GCC 12.2.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import xopen, os >>> f = xopen.xopen("file.zst") >>> f.seek(0, os.SEEK_END) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/nfs/site/home/<user>/pyenv3.11.1/lib/python3.11/site-packages/xopen/__init__.py", line 476, in seek return self._file.seek(offset, whence) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ io.UnsupportedOperation: underlying stream is not seekable ```