Fix: namespace inheritance

[dapper91]

• an exception is raised if namespace alias is not found in the namespace map.
• a child model inherits the parent namespace map.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.08%. Comparing base (2d2734c) to head (bb30d8b).

Files with missing lines	Patch %	Lines
pydantic_xml/utils.py	71.42%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #255      +/-   ##
==========================================
- Coverage   92.16%   92.08%   -0.08%     
==========================================
  Files          29       29              
  Lines        1671     1680       +9     
==========================================
+ Hits         1540     1547       +7     
- Misses        131      133       +2     

Flag	Coverage Δ
unittests	92.08% <81.81%> (-0.08%)	⬇️

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