Fix the record field name validator regex

[pyrco]

The original regex allowed a literal "[]" at the end of a name, which breaks using the name as Python identifier.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.95%. Comparing base (c17dc6b) to head (c699a41).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
+ Coverage   83.92%   83.95%   +0.02%     
==========================================
  Files          34       34              
  Lines        3466     3466              
==========================================
+ Hits         2909     2910       +1     
+ Misses        557      556       -1     

Flag	Coverage Δ
unittests	83.95% <100.00%> (+0.02%)	⬆️

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