chore: use new syntax for union types

[cailloumajor]

Fixes #145

[codecov]

Codecov Report

Merging #155 (f4f9134) into main (a84d7be) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
- Coverage   96.17%   96.16%   -0.01%     
==========================================
  Files           7        7              
  Lines         523      522       -1     
  Branches       91       91              
==========================================
- Hits          503      502       -1     
  Misses         17       17              
  Partials        3        3              

Flag	Coverage Δ
unittests	96.16% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/opcua_webhmi_bridge/frontend_messaging.py	100.00% <100.00%> (ø)
src/opcua_webhmi_bridge/influxdb.py	100.00% <100.00%> (ø)
src/opcua_webhmi_bridge/messages.py	100.00% <100.00%> (ø)

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