Fix PytestReturnNotNoneWarning

Fix PytestReturnNotNoneWarning

```
test_examples.py::test_printcap
  /usr/local/lib/python3.9/dist-packages/_pytest/python.py:199: PytestReturnNotNoneWarning: Expected None, but test_examples.py::test_printcap returned ((7, 38), {'FUSE_CAP_READDIRPLUS_AUTO', 'FUSE_CAP_ASYNC_DIO', 'FUSE_CAP_SPLICE_READ', 'FUSE_CAP_CACHE_SYMLINKS', 'FUSE_CAP_IOCTL_DIR', 'FUSE_CAP_NO_OPENDIR_SUPPORT', 'FUSE_CAP_NO_OPEN_SUPPORT', 'FUSE_CAP_POSIX_LOCKS', 'FUSE_CAP_READDIRPLUS', 'FUSE_CAP_POSIX_ACL', 'FUSE_CAP_ATOMIC_O_TRUNC', 'FUSE_CAP_SPLICE_MOVE', 'FUSE_CAP_EXPORT_SUPPORT', 'FUSE_CAP_FLOCK_LOCKS', 'FUSE_CAP_EXPLICIT_INVAL_DATA', 'FUSE_CAP_EXPIRE_ONLY', 'FUSE_CAP_DONT_MASK', 'FUSE_CAP_WRITEBACK_CACHE', 'FUSE_CAP_AUTO_INVAL_DATA', 'FUSE_CAP_PARALLEL_DIROPS', 'FUSE_CAP_SPLICE_WRITE', 'FUSE_CAP_ASYNC_READ'}), which will be an error in a future version of pytest.  Did you mean to use `assert` instead of `return`?
    warnings.warn(
```