fix: named-tuple core schema support

[dapper91]

• fix: added named-tuple core schema support (added in pydantic 2.14 release).
• tests: importlib replaced by runpy for testing examples.
• python 3.15 support added.
• python 3.8 / 3.9 support dropped.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.12%. Comparing base (17aef51) to head (e6abbb4).
⚠️ Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
pydantic_xml/serializers/factories/call.py	0.00%	1 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #331      +/-   ##
==========================================
- Coverage   92.22%   92.12%   -0.11%     
==========================================
  Files          31       31              
  Lines        1788     1765      -23     
==========================================
- Hits         1649     1626      -23     
  Misses        139      139              

Flag	Coverage Δ
unittests	92.12% <92.30%> (-0.11%)	⬇️

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