Make leap_day=True default for PSM3

[AdamRJensen]

• [x] Closes #1481
• [x] I am familiar with the contributing guidelines
• [ ] Tests added
• ~~[ ] Updates entries in docs/sphinx/source/reference 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`).
• [x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [ ] Pull request is nearly complete and ready for detailed review.
• [x] Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

This PR should first be merged for pvlib 0.11.0!

This PR changes the default value of the leap_day parameter in pvlib.iotools.get_psm3 from False to True and removes the deprecation warning added in #1511. The reason for the change is that it seems more intuitive to include leap days when requesting non-TMY time series, which is also the default behavior of the remaining iotools functions.