Fix numpy deprecation warning when running tests in test_metropolis.py

[asifzubair]

Description

This PR is an attempt to address the numpy deprecation warning (#7369). Previous attempts to make the fix (ex., #7514) focussed on checking for scalars in pymc/backends/ndarray.py. Following this hint, the present PR instead checks which offending stat variable is not respecting the scalar requirement and modifies that.

After the fix, we can see that no warnings are seen:

(pymc-dev) jovyan@75886b6f8645:~/work$ python -m pytest tests/step_methods/test_metropolis.py 
=== test session starts ===
 ...

tests/step_methods/test_metropolis.py .............................................                                               [100%]

==== 45 passed in 259.02s (0:04:19) ===

Related Issue

• Closes Numpy deprecation warning issued from within test_metropolis.py #7369
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

---

📚 Documentation preview 📚: https://pymc--7871.org.readthedocs.build/en/7871/

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (dc7cfee) to head (405fd59).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7871      +/-   ##
==========================================
+ Coverage   88.25%   92.94%   +4.69%     
==========================================
  Files         116      116              
  Lines       18845    18845              
==========================================
+ Hits        16631    17515     +884     
+ Misses       2214     1330     -884     

Files with missing lines	Coverage Δ
pymc/step_methods/metropolis.py	93.14% <ø> (ø)

... and 1 file with indirect coverage changes

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Congrats on merging your first pull request! 🎉 We here at PyMC are proud of you! 💖 Thank you so much for your contribution 🎁

[ricardoV94]

Thanks @asifzubair