chore: deprecate assignUniqueLabels

[stevenhua0320]

@sbillinge ready to review.
Now the test for this function would give deprecationWarning shown below.

(diffpy.structure) ~/dbs/diffpy.structure git:[deprecate-assignUniqueLabels]
pytest
============================================================================================================== test session starts ==============================================================================================================
platform darwin -- Python 3.14.3, pytest-9.0.2, pluggy-1.6.0
rootdir: /Users/huarundong/dbs/diffpy.structure
configfile: pyproject.toml
collected 138 items                                                                                                                                                                                                                             

tests/test_atom.py ..                                                                                                                                                                                                                     [  1%]
tests/test_lattice.py ............                                                                                                                                                                                                        [ 10%]
tests/test_loadstructure.py ......                                                                                                                                                                                                        [ 14%]
tests/test_p_cif.py .....................                                                                                                                                                                                                 [ 29%]
tests/test_p_discus.py .....                                                                                                                                                                                                              [ 33%]
tests/test_p_pdffit.py .........                                                                                                                                                                                                          [ 39%]
tests/test_parsers.py .............                                                                                                                                                                                                       [ 49%]
tests/test_spacegroups.py .....                                                                                                                                                                                                           [ 52%]
tests/test_structure.py ........................................                                                                                                                                                                          [ 81%]
tests/test_supercell.py ...                                                                                                                                                                                                               [ 84%]
tests/test_symmetryutilities.py .....................                                                                                                                                                                                     [ 99%]
tests/test_version.py .                                                                                                                                                                                                                   [100%]

=============================================================================================================== warnings summary ================================================================================================================
tests/test_structure.py::TestStructure::test_assignUniqueLabels
  /Users/huarundong/dbs/diffpy.structure/tests/test_structure.py:128: DeprecationWarning: 'diffpy.structure.Structure.assignUniqueLabels' is deprecated and will be removed in version 4.0.0. Please use 'diffpy.structure.Structure.assign_unique_labels' instead.
    self.stru.assignUniqueLabels()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================================================================================== 138 passed, 1 warning in 0.63s =========================================================================================================

[sbillinge]

I will merge this but @stevenhua0320 and @cadenmyers13 please can you see my comment

[sbillinge]

Why is this no news. Don't we need to list what we deprecate and what we add?

[sbillinge]

I don't think we need to do this. This test will start failing when we remove the function so these will be easy to find and remove.

[stevenhua0320]

@sbillinge So, what we need to contain here? should we maintain the original docstring here?

[sbillinge]

Actually, I can't merge because of the news. I am afraid that I may have merged som no-news's that should have had news in them. Please can you take a look @stevenhua0320 and add PRs if I did?

[stevenhua0320]

@sbillinge It's safe right now since nothing deprecation's PR has been merged yet. I would now change the news and see anything that we need edit here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.93%. Comparing base (1b565c8) to head (2a1f30e).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #158   +/-   ##
=======================================
  Coverage   98.93%   98.93%           
=======================================
  Files          13       13           
  Lines        1878     1884    +6     
=======================================
+ Hits         1858     1864    +6     
  Misses         20       20           

Files with missing lines	Coverage Δ
tests/test_structure.py	99.78% <100.00%> (+<0.01%)	⬆️

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