Fix expanding altreps with attributes

[trossi]

References to issues or other PRs

See discussion in #45 (comment).

Describe the proposed changes

This PR will fix setting altrep attributes when expand_altrep=True (default). If altrep's attributes are NILVALUE, then attributes are not set in the expanded object.

Additional information

The test files are generated by using the example here.

Checklist before requesting a review

• I have performed a self-review of my code
• The code conforms to the style used in this package (checked with Ruff)
• The code is fully documented and typed (type-checked with Mypy)
• I have added thorough tests for the new/changed functionality