Fix: Update repeated field checks to use is_repeated property

[pdxlocations]

Update protobuf field-inspection logic to use is_repeated instead of label where needed.

I have not tested for backwards compatibility.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.84%. Comparing base (d0ccb1a) to head (b003214).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/__main__.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #908      +/-   ##
==========================================
+ Coverage   59.82%   59.84%   +0.01%     
==========================================
  Files          24       24              
  Lines        4329     4333       +4     
==========================================
+ Hits         2590     2593       +3     
- Misses       1739     1740       +1     

Flag	Coverage Δ
unittests	59.84% <57.14%> (+0.01%)	⬆️

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[pdxlocations]

Tested with older protobuf dependencies and we'd need bump to >=6.31.0 or add a fallback.

[pdxlocations]

Helper function added to test for is_repeated and fallback to label if not present.
Tested with the current minimum dependency of 4.4.21 3.20.3 and up to 7.34.0.