Fix warning message for num_threads in pvlib.spa

[kandersolar]

• [ ] Closes #xxxx
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• Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
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• [ ] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
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The numba SPA implementation uses multithreading by default. When the specified number of threads is larger than the number of given timestamps, a warning is emitted indicating that the number of used threads is being reduced to match the input timestamps. However, the message mixes two styles of interpolation (old %s style and .format style) and thus produces a faulty message with the interpolation placeholder still present:

UserWarning: The number of threads is more than the length of the time array. Only using %s threads.

This PR fixes the interpolation so that it would then look something like this:

UserWarning: The number of threads is more than the length of the time array. Only using 3 threads.