Fix loading files with coordinates that have non-string units

[schlunma]

🚀 Pull Request

Description

The function _is_lat_lon fails with an AttributeError if the coordinate variable has non-string units.

This PR adapts this function so it can handle this case.

Fixes #6505

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.87%. Comparing base (3fe8169) to head (d419ad9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6506   +/-   ##
=======================================
  Coverage   89.87%   89.87%           
=======================================
  Files          90       90           
  Lines       23908    23908           
  Branches     4459     4459           
=======================================
  Hits        21487    21487           
  Misses       1669     1669           
  Partials      752      752           

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[bjlittle]

@schlunma LGTM 👍