Swap pydantic for dataclasses

[khaeru]

This PR uses dataclasses from the Python standard library for many of the convenience features previously used from pydantic, namely generating __init__() methods automatically for the large number of classes in the SDMX Information Model.

Some effects:

• The package no longer depends on pydantic; we no longer need to maintain pydantic compatibility code that may break.
• Validation overhead is reduced where not necessary. In particular, the switch allowed to identify and fix the few instances where internal code usage was not satisfying type expectations.
• Expansion to SDMX 3.0 (Implement/ensure compat with SDMX 3.0 #87) will be easier.
• Performance may differ, but since we do not yet have performance tests (Use pytest-profiling and describe performance tests #49) the effect is not clear; anecdotally the test suite seems to take 50% as long to run.

Internal changes:

• Remove .util.{BaseModel,Field,validate_dictlike,validator}.
• Remove DictLike.{__get_validators__,_validate_whole}.
• Organize tests of .model classes in test files that parallel the structure of the module.

PR checklist

• Address remaining TODOs inline, plus
  • Tidy reuse of __post_init__, __eq__, and __hash__ from parent classes.
  • Remove Key._fast() and usage.
• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

Merging #128 (92a309e) into main (dae5e79) will decrease coverage by 2.18%.
The diff coverage is 99.36%.

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
- Coverage   98.10%   95.93%   -2.18%     
==========================================
  Files          72       73       +1     
  Lines        5446     5602     +156     
==========================================
+ Hits         5343     5374      +31     
- Misses        103      228     +125     

Impacted Files	Coverage Δ
sdmx/reader/xml.py	97.61% <ø> (-0.45%)	⬇️
sdmx/source/bbk.py	52.94% <ø> (-36.54%)	⬇️
sdmx/tests/reader/test_reader_xml.py	100.00% <ø> (ø)
sdmx/tests/writer/test_writer_xml.py	100.00% <ø> (ø)
sdmx/writer/xml.py	97.57% <83.33%> (-0.77%)	⬇️
sdmx/util.py	98.51% <96.77%> (-1.49%)	⬇️
sdmx/model/v21.py	99.83% <99.23%> (+1.20%)	⬆️
sdmx/client.py	94.05% <100.00%> (-3.79%)	⬇️
sdmx/message.py	100.00% <100.00%> (ø)
sdmx/model/common.py	100.00% <100.00%> (+0.94%)	⬆️
... and 12 more

... and 13 files with indirect coverage changes