Refactor the Deserialization API into an Assertion API

[bartfeenstra]

This fixes #1567

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 91.76471% with 28 lines in your changes missing coverage. Please review.

Project coverage is 95.94%. Comparing base (b53f91b) to head (67b8d0a).
Report is 329 commits behind head on 0.4.x.

Files with missing lines	Patch %	Lines
betty/assertion/__init__.py	90.77%	12 Missing and 7 partials ⚠️
betty/tests/assertion/__init__.py	81.39%	5 Missing and 3 partials ⚠️
betty/extension/gramps/gui.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            0.4.x    #1652      +/-   ##
==========================================
- Coverage   95.95%   95.94%   -0.01%     
==========================================
  Files         205      205              
  Lines       19224    19234      +10     
  Branches     3602     3602              
==========================================
+ Hits        18446    18454       +8     
- Misses        526      527       +1     
- Partials      252      253       +1     

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