Prefer log.warning("%s", ...) to log.warning("%s" % ...).

[anntzer]

This is suggested by the stdlib docs to avoid paying the cost of the
formatting if the message doesn't actually get emitted. It's marginally
shorter too, especially when ... is surrounded by parentheses.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way