Move the Requirement API to the package root

[bartfeenstra]

This fixes #1424

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 92.13483% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 93.06%. Comparing base (e8be8b9) to head (e037d9e).
Report is 1 commits behind head on 0.3.x.

Files	Patch %	Lines
betty/app/extension/requirement.py	0.00%	4 Missing ⚠️
betty/requirement.py	96.38%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            0.3.x    #1426      +/-   ##
==========================================
- Coverage   93.11%   93.06%   -0.05%     
==========================================
  Files          86       87       +1     
  Lines        9532     9537       +5     
==========================================
  Hits         8876     8876              
- Misses        656      661       +5     

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