fix: include p-z in _is_single_cell_widths regex

[TomJGooding]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Please describe your changes here. If this fixes a bug, please link to the issue, if possible.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.27%. Comparing base (9c74f03) to head (250127d).
Report is 94 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3534      +/-   ##
==========================================
+ Coverage   98.25%   98.27%   +0.01%     
==========================================
  Files          74       74              
  Lines        8072     8095      +23     
==========================================
+ Hits         7931     7955      +24     
+ Misses        141      140       -1     

Flag	Coverage Δ
unittests	98.27% <100.00%> (+0.01%)	⬆️

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[TomJGooding]

Closed in favour of #3535