Corrected future warning in solarposition.py

[YoannUniKS]

• corrected code to avoid: "casting datetime64[ns, Europe/Berlin] values to int64 with .astype(...) is deprecated and will raise in a future version. Use .view(...) instead."

• Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

[wholmgren]

Thanks @YoannUniKS! Feel free to add your name/username to the 0.9.0.rst whats new file or to skip it. I'll merge in a few days unless I hear otherwise.