Address minor numpy deprecation

[kandersolar]

• [ ] Closes #xxxx
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• Tests added
• [ ] Updates entries in docs/sphinx/source/reference for API changes.
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Addresses this deprecation warning shown in recent test runs:

tests/test_clearsky.py::test_detect_clearsky_irregular_times
/home/runner/work/pvlib-python/pvlib-python/tests/test_clearsky.py:666: DeprecationWarning: The 'generic' unit for NumPy timedelta is deprecated, and will raise an error in the future. This includes implicit conversion of bare integers (e.g. + 1). Please use a specific unit instead.
times[0] += 10**9

https://github.com/pvlib/pvlib-python/actions/runs/34241529820/job/102112640274#step:9:77