tests: modernize assertions in test_persistencelength (ref #3743)

[palak22291]

Summary

This PR modernizes deprecated NumPy testing assertions in testsuite/MDAnalysisTests/analysis/test_persistencelength.py as part of issue #3743.

Changes

• Replaced scalar uses of assert_almost_equal() with pytest.approx()
• Removed the deprecated assert_almost_equal import

Verification

• Ran:

  pytest testsuite/MDAnalysisTests/analysis/test_persistencelength.py

• Result: all 30 tests passed successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.85%. Comparing base (941d679) to head (476dd6e).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5407   +/-   ##
========================================
  Coverage    93.85%   93.85%           
========================================
  Files          182      182           
  Lines        22509    22509           
  Branches      3202     3202           
========================================
  Hits         21125    21125           
  Misses         922      922           
  Partials       462      462           

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