The PSM3 API endpoints have been removed:

• Lots of random "Data processing failure." from NSRDB API NREL/developer.nrel.gov#382 (comment)
• NSRDB - Remove deprecated NREL/developer.nrel.gov#424

As a result, the test_psm3.py tests are failing. Example: https://github.com/pvlib/pvlib-python/actions/runs/18792512909/job/53625869585

I think there's nothing for it but for us to remove get_psm3. It's already unusable, so I see little point in deprecating it or replacing it with a "you need to migrate to the PSM4 functions" shim.

What to do with read_psm3? Of course it still works fine, but given that read_psm4 is functionally identical (I think?), I suggest we just delete psm3.py entirely.