🐛 Fix cyclic imports

[ubmarco]

This PR fixes circular SN imports by moving exceptions.py to the root.

A test case was added to prevent this in future.

Note

Ruff might support this in future: astral-sh/ruff#2914

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.56%. Comparing base (4e10030) to head (1898050).
Report is 129 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/config.py	0.00%	1 Missing ⚠️
sphinx_needs/exceptions.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1443      +/-   ##
==========================================
+ Coverage   86.87%   88.56%   +1.68%     
==========================================
  Files          56       61       +5     
  Lines        6532     7242     +710     
==========================================
+ Hits         5675     6414     +739     
+ Misses        857      828      -29     

Flag	Coverage Δ
pytests	88.56% <94.87%> (+1.68%)	⬆️

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